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Neurobiological effects of odors and air quality

during sleep

Thomas van Bergeyk, s2782200 Supervisor: Marijke Gordijn

University of Groningen Faculty of Science and Engineering

Chronobiology research 15-12-2018

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Abstract In the human nose there are around 1000 different subtypes of olfactory receptors, which can detect a great amount of olfactory molecules. The human nose can detect two different kind of odors. The first kind: purely olfactory odors, only activate olfactory receptors and affect the olfactory neurons. The second kind: trigeminal odors, activate the trigeminal receptors as well as the olfactory receptors and are often considered as irritants. During wakefulness, olfactory stimulation is one input to the sensory system that elicits strong effects on emotions and memory. Whether olfactory stimulation elicits the same strong effects on emotions and memory during sleep remains not fully clear. This leads to the research question, what are the neurobiological effects of odors and air quality during sleep? The neurobiological effects of odors during sleep are that purely olfactory odors increases vigor in the morning as well as slow wave sleep. Olfactory stimulation could improve objective sleep quality by increasing sleep efficiency. Odors can also be used to make novel associations and improve memory consolidation during sleep. The neurobiological effects of air quality during sleep are that subjective sleep quality increases when the indoor air quality is good. The effective usage of a good ventilation system can help with good indoor air quality.

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Table of contents Abstract 2 Table of contents 3 Introduction 4 Sleep and sleep quality 5 Olfactory stimulation 5 Effect of air quality on sleep 6 Subjective and objective neurobiological olfactory stimulation effects on sleep 7 Associative learning with olfactory stimulation during sleep 10 Discussion 15 References 17

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Introduction In the human nose there are around 6 million olfactory neurons. Axons of olfactory neurons project to the olfactory bulb. Each primary olfactory neuron sends an olfactory rod to the surface of the olfactory epithelium, the location and structure of the olfactory epithelium can be seen in figure 1. Around 6 to 8 olfactory cilia stick out of the end of a single olfactory rod. These olfactory cilia contain receptor molecules that can hold the molecules of odors. (Coren, Ward, & Enns, 2004). There are around 1000 different types of olfactory receptors which hold different molecules of odors and in turn activate olfactory neurons (Nagel & Wilson, 2011). Activation of these neurons leads to detection and eventually even perception of odors (Secundo, Snitz, & Sobel, 2014). Wiring in the olfactory bulb appears to be directly linked with behavior via cortical projection towards the amygdala (Imai, 2014). The olfactory bulb is linked with the amygdala via the piriform cortex. Olfactory information that is processed and send to the amygdala relates odors with emotions. Odors can also be linked with certain memories which on its turn can be strongly linked to an emotion. An odor elicits an emotion via a pathway to the amygdala and it is linked with episodic memory via the hippocampus, which receives olfactory information from the amygdala and the entorhinal cortex (Kadohisa, 2013). During wakefulness, olfactory stimulation is one input to the sensory system that elicits strong effects on emotions and memory. During slow wave sleep the piriform cortex becomes hyporesponsive for signals from the olfactory bulb, while functional connectivity between this cortex and other cortices is strengthened. Functional connectivity is the interaction between anatomical distinct brain regions that share functional properties, and it is assessed using fMRI. This may lead to an enhancement in odor memory consolidation during sleep (Dylan C Barnes & Wilson, 2014; Wilson, Hoptman, Gerum, & Guilfoyle, 2011). Since sleep is thought to be important for learning and memory, the role of odors and, maybe more in general, characteristics of air quality during sleep on wellbeing and memory consolidation is an interesting topic. This leads to the research question, what are the neurobiological effects of odors and air quality during sleep?

Figure 1 The location and structure of the olfactory epithelium (Bear, Connors, & Paradiso, 2007).

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Sleep and sleep quality During sleep people elicit different behavior than during wakefulness. Behaviors that are elicited during sleep include a lack of mobility, slow eye movements, reversible unconscious state, reduced reaction to external stimuli and many more (Chokroverty, 2010). Different sleep stages are measured via electroencephalography (EEG), this method records electrical activity in the brain. Sleep can be divided into two states. Non rapid eye movement (NREM) sleep, which consists of three stages, NREM1, NREM2 and NREM3. NREM1 is considered as a transition stage between wakefulness and sleep. During NREM2 sleep-spindles and K-complexes are visible on an EEG, these are types of brain activity. Slow wave sleep occurs during NREM3 and is the so called deep sleep. Slow wave sleep is characterized by slow oscillations of brain activity. The other state of sleep is rapid eye movement (REM) sleep. EEG activity of REM sleep is very similar to EEG activity during wakefulness. The difference is that during REM sleep muscle activity is mostly absent, people have a low consciousness and are not awake (Equihua-Benítez, Guzmán-Vásquez, & Drucker-Colín, 2017). To determine the quality of sleep, different parameters are used. Data on sleep quality can be acquired objectively, via the use of actograms and EEG, electromyography (EMG) and electrooculography (EOG) during sleep. In this way, the behavioural and physiological changes which occur during sleep are measured. Subjective sleep quality is measured via the use of questionnaires like the Pittsburgh Sleep Quality Index (PSQI) (Buysse et al., 1989) and the Groningen Sleep Quality scale (Mulder-Hajonides van der Meulen et al., 1980). In the PSQI seven components of sleep are assessed: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medication, and daytime dysfunction. For each component a score is given and subjective sleep quality is determined by adding the scores. Higher scores indicate a lower sleep quality. Sleep efficiency (SE) is one of the most used quantitative measures to evaluate sleep quality (Jung, Lee, Jeong, & Park, 2017). SE is a ratio determined by the total sleep time divided by the total time spent in bed. Åkerstedt and colleagues (1994) investigated in a longitudinal and intraindividual design the meaning of subjectively good sleep. They found that SE is more related to good sleep than sleep continuity variables like sleep latency, final wake time and wake after sleep onset. A high sleep efficiency and a good sleep quality was found in individuals with high exercise activity (Gubelmann, Heinzer, Haba-Rubio, Vollenweider, & Marques-Vidal, 2018; Kline et al., 2013). Good subjective sleep quality (PSQI≤5) has also been correlated with a significant higher SE (P < 0.001) and a significant shorter sleep latency (P < 0.001) in daytime-working adults (Soehner, Kennedy, & Monk, 2011).

Olfactory stimulation To determine the effect of olfactory stimulation, multiple odors must be used, since odors are perceived differently. There is a subjective distinction between generally pleasant odors, like lavender oil, peppermint oil, jasmine and vanillin and generally unpleasant odors like rotten fish, undiluted vetiver oil and ammonium sulfide. Another distinction between odors is that odors can be olfactory, which are odors that only bind to olfactory receptors. Odors can also be trigeminal, trigeminal odors can bind to trigeminal receptors as well as olfactory receptors and are often considered irritants. When a trigeminal odor is received by a trigeminal receptor it sends a signal primarily to the trigeminal nerve (cranial nerve V) (Purves D, Augustine GJ, Fitzpatrick D, et al., 2001). Vanillin is an example of a purely olfactory odor and vinegar of a trigeminal odor. Odors could be experienced differently by each individual. Olfactory receptors are encoded by approximately 400 functional genes and approximately 600 putative pseudogenes in every person’s genome (Buck & Axel, 1991; Gilad & Lancet, 2003). The sequence and functionality differ for each individual, therefore it has been argued that the variation in the genes contribute to a certain repertoire of olfactory receptors which differs for each human nose (Logan, 2014). Although there is genetic variation, the gross olfactory perception remains similar across humans. The reason why humans have 1000 different subtypes of olfactory receptors is not yet fully known. As an example, Fleischmann and colleagues (2008) created mice in which more than 95% of the sensory neurons

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express a single olfactory receptor. If it would be important to have 1000 different subtypes of olfactory receptors, these mice would be heavily impaired in their olfactory perception. However these mice could smell almost every odorant. There was only one olfactory disability in these mice, they could not detect acetophenone, which is the ligand that binds to a over-expressed receptor subtype. It would appear that the 1000 different subtypes are not necessary to perceive most odors, the average perception of an odor can serve as a decent estimate of what any given individual would say about the odor (Secundo et al., 2015).

Effect of air quality on sleep Something that is related to odors, but could have a separate effect on sleep quality and well-being during wakefulness is the indoor air quality, which is usually measured by CO2 concentration, the amount of ventilation, air temperature, air humidity, viable mold, wind speed and airborne dust levels (Jurado, Bankoff, & Sanchez, 2014).

Strøm-Tejsen and colleagues (2016) performed two different experiments to test the effect of bedroom air quality on sleep and the next-day performance. Both experiments were single-blind and were in a random order. The two different experiments consist of an intial pilot experiment and a main experiment. All the data was gathered from participants that lived in the same dormitory. In the pilot, participants were exposed to two experimental conditions, either with the window open or closed. Each condition lasted 1 week. In the main experiment participants were also exposed to two experimental conditions. Here an inaudible fan was placed that let outdoor air mechanicly in (ventilation) or out (no ventilation). Each condition lasted a week. For next-day performance a logical thinking test was administered (Baddeley’s test of grammatical reasoning), also the Groningen Sleep Quality (GSQ) scale was filled in.

They found that during the ventilation condition the average CO2 concentration was 835 ppm in the main experiment and 660 ppm in the pilot. During no ventilation condition the average CO2 concentration was 2395 ppm in the main experiment and 2585 ppm in the pilot. These results are also reflected in the subjective measurements. Participants reported that the bedroom air was fresher during the ventilation condition in both experiments (table 1). SE is significantly higher during the ventilation condition in the main experiment (P < 0.0494). For the pilot experiment there was a tendency for SE to be higher during the ventilation condition (P < 0.0736). For the GSQ scale no significant result was obtained between conditions for either experiment (two-tail P-values were 0.1080 for the pilot and 0.0664 for the main experiment). They argue that both experiments can be regarded as two individual tests of the same directional hypothesis, so they combine the one-tail P-values yielding a significant result (P < 0.02). No actual scores were given for the GSQ scale, but it is stated that the responses on the GSQ scale improved during the ventilation condition. For the logical thinking test significant results (P < 0.02) were found after combining the P-values for both experiments again. Combining these objective and subjective results, this study concludes that the sleep quality is improved under the ventilation condition compared to the no ventilation condition. In this study the outdoor air was clean, so when this is not the case, like in certain bigger cities a proper filter has to be installed. There was no significant diference in room temperature between the conditions.

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Table 1 Subjective assessments of freshness of air for all subjects from the pilot and main experiment (modified from: Strøm-Tejsen et al., 2016).

Subjective and objective neurobiological olfactory stimulation effects on sleep Odors have an effect on sleep. One of these effects is that odor is replayed in the brain during sleep. Sleep dependent odor replay is a component of memory consolidation, it means that recently acquired information of an odor is replayed during sleep. Replay occurs spontaneously during sleep in the hippocampus. It can also be imposed by presenting sensory stimuli during sleep. During slow wave sleep, sensory cortices like the olfactory and auditory cortex are hyporesponsive to external stimuli (Issa & Wang, 2011; Murakami, Kashiwadani, Kirino, & Mori, 2005). This hyporesponsiveness causes less interference and thus gives more room for replay to consolidate memory. Even though sensory cortices are hyporesponsive, presentation of contextual cues during sleep enhances memory consolidation (Bendor & Wilson, 2012; B Rasch, Büchel, Gais, & Born, 2007). Replay that is imposed during slow wave sleep results in the strengthening of memory consolidation (D. C. Barnes & Wilson, 2014). Arzi and colleagues (2009) studied the effects of odors on sleep and used a double-blind randomly controlled trial where they presented randomized 1 of 4 odors to a participant with sleep apnea during NREM2 sleep stage. Sleep apnea is a disorder where breathing during sleep is not normal. It is characterized by repetitive cessation or decreased amplitude of breathing (AASM, 1999). Besides having clear clinical consequences, it also causes arousals or waking (Chesson et al., 1997). Participants with sleep apnea were chosen, because in this study they wanted to see whether odors have an effect on the respiratory patterns. The odors that were used were either purely olfactory odors, vanillin (3% v/v vanillin diluted in water) or ammonium sulfide (1% v/v ammonium sulfide diluted in water). Or the odors were mildly trigeminal, undiluted lavender oil or undiluted vetiver oil. This group used a nasal mask. How this mask precisely works is explained in (Johnson & Sobel, 2007; Sobel et al., 1997). 20 minutes after it was determined that the subject had entered stage 2 sleep for the first time, the experimental protocol started. Every 9, 12 or 15 minutes (randomized) the olfactometer generated a 5-, 10- or 20-s (randomized) odor stimulus. This resulted in 21-37 odor stimuli per night. They found no significant effect, neither an increase nor a decrease on the number of arousals (P < 0.12) and awakenings (P < 0.16) with any odors. There was however a trend for lavender oil to reduce the frequency of awakenings (P < 0.084). If odors could have any effect on arousals and awakenings it would be a reducing effect.

In a single-blind between subjects experiment performed in a random order by Badia and colleagues (1990), either peppermint oil (0,26 mg/liter) or simple air was given to the participants during sleep. The number of microswitch closures were higher during the fragrance trials than the non-fragrance trials. A microswitch closure means that during the fragrance and non-fragrance trials if they consciously perceived the smell, participants woke up and pressed the microswitch. These

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results are drawn from data where the odor is presented during NREM2 sleep. There were significantly more microswitch closures during fragrance trial (12,43%) compared to the non-fragrance trials (1,27%, P < 0.05). Different percentages given are a percentage of a stimuli eliciting a response. There was a significant higher percentage of EEG speeding during the fragrance trials (15,6%) than during the non-fragrance trials (3,85%, P < 0.05). EEG speeding is defined in this study as a high frequency EEG burst (similar to wake) lasting less than 10 seconds. A time-of-night effect was found, the participants were less responsive to the odor in the latter third part of the night. The night was divided into three equal parts for each individual separately depending on the time of the first and last epoch. This is true for the microswitch closures, where during the fragrance trials in the first and second part of the night 16,74% and 16,19% was observed respectively. This compared to 4,37% of the last and third part of the night. A two-way ANOVA revealed a significant effect of a greater responsivity on the fragrance in the beginning of the night compared to the latter part indicating that subjects could detect the odor better in the first 2 parts of the night (P < 0.05). This study assumes that detection of olfactory stimuli is due olfactory chemoreception. However they cannot rule out the possibility of trigeminal chemoreception. In a controlled cross over trial in a study done by Goel and colleagues (2005) either undiluted lavender oil or distilled water was used before “lights off” at 24:00 h in a within subjects design. Subjects had to get out of bed at 08:00 h. Whether a subject received undiluted lavender oil or distilled water was determined at random. 31 subjects participated in this experiment, 16 male and 15 female (overall mean age + SD, 20.5 + 2.4 yr) subjects stayed in a sleep laboratory for 3 consecutive nights, the first night was considered an adaptation session. On the second night either undiluted lavender oil or distilled water was presented from 23:10 until 23:40 h. Presentation of the odor occurred for 2 minutes each 10 minute period. This particular oil was validated as a sedative externally (Goel & Grasso, 2004). No information on the volume of the vials in which the undiluted lavender oil was present was given. To assess vigor, this study used the Profile of Mood States Questionnaire (POMS) (McNair, Lorr, & Droppleman, 1992). Undiluted lavender oil significantly (P < 0.005) increased duration spend in slow wave sleep. There was a higher percentage slow wave sleep during the sleep period time (which is the duration from sleep onset to the end of sleep) after the lavender trial (6,9% ± 4,1%) compared to the distilled water trial (5,7% ± 4,2%). Values are means percentage ± SD. This effect was significant in the first half of the night (P < 0.05). With values of 13,3% ± 7,7% for the lavender trial vs 11,5% ± 8,5% for the distilled water trial. Results are almost significant (P = 0.08) in the latter half of the trial. With values of 0,9% ± 0,4% for the lavender trial vs 0,4% ± 0,5% for the distilled water trial. Although there was no significant effect found in the latter half of the night, the trend that is shown is still in the same direction as the significant result in the first half of the night. The effect in the first half of the night was predominantly found during the first NREM-REM sleep cycle. These results suggests that the effect of lavender oil, in the last period before going to sleep, is rather fast, with deeper sleep in the beginning after sleep onset and doesn’t have a big effect after the first NREM-REM sleep cycle. Vigor was assessed via the POMS, and the only significant difference was found at 8:00 h (P < 0.05). Vigor was higher here in the lavender session than in the control. Vigor was tested 4 times in total, 3 times before sleep (23:00 h, 23:12 h and 23:42 h) and once in the morning (8:00 h) (figure 2). How long the increase of vigor lasts in the morning was not tested. Via the PSG measures sleep efficiency was also measured, this did not differ significantly (96,1% ± 5,0% for the lavender trial vs 95,5% ± 5,0% for the distilled water trial). The findings of the subjective and objective neurobiological effects of olfactory stimulation on sleep are summarized in table 2.

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Figure 2 Vigor POMS scores across assessment time points for the stimulus and control nights (mean + SD). Significantly greater than the control night, p , 0.05. The stimulus or control were presented for 2 minutes each 10 minutes starting at 23:10h and ending at 23:42h (this figure is based of figure 3 from Goel et al., 2005).

Table 2 Results of the subjective and objective neurobiological effects of olfactory stimulation on sleep.

Author and year

Blinding / study design

Study objective Primary findings

Arzi et al., 2009

Double-blind randomly controlled

Evaluate effect of odor on arousals/awakenings during sleep

No significant effect was found. However a trend towards a reduction in frequencies of awakenings by the use of lavender oil was found.

Badia et al., 1990

Single-blind between subjects

Evaluate effect of peppermint oil vs normal air presentation on human’s reaction during sleep

Significantly more microswitch closures as well as a higher percentage of EEG speeding during the peppermint oil trial. A time-of-night effect was found, subject there were more microswitch closures in the first 2 thirds of the night.

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Goel et al., 2005

Controlled crossover experiment with a within subjects design

Evaluate effect of undiluted lavender oil vs distilled water presentation on night-time objective sleep

Higher percentage slow wave sleep during the undiluted lavender oil trial. A time-of-night effect was found, in the first NREM-REM sleep cycle a higher percentage slow wave sleep was found. Subjectively, vigor is significantly increased after sleep in the undiluted lavender oil trail compared to the distilled water trial.

Associative learning with olfactory stimulation during sleep Studies have been done on whether associative learning can be promoted with olfactory stimulation during sleep. In a single-blind randomized controlled trial study done by Arzi and colleagues (2012) they presented the participants with trials throughout their night during sleep. Partial cue conditioning was used, participants were presented with a tone (400 Hz or 1200 Hz) which was followed by either a pleasant (shampoo or deodorant), unpleasant (carrion or rotten fish) or no odor during sleep. Both the concentration and volume of these odors are not given. It is stated that the concentration is low and non-trigeminal. In general a more pleasant odor elicits a stronger sniff response than an unpleasant one (Bensafi et al., 2003). Participants were not aware of the test during sleep. The same nasal mask was used as in another study performed by this group as stated previously (Arzi et al., 2009). During sleep the sniff volume was greater following pleasant odors compared to unpleasant odors (pleasant odor, 0.96 ± 0.09 normalized volume units (nvu); unpleasant odors, 0.90 ± 0.12 nvu; P < 0.001; figure 3a). Even in sleep, pleasant odors give a stronger sniff response than unpleasant odors. In the trial where only a tone was presented, without a following odor, that tone which was previously paired with a pleasant odor during sleep resulted in a significantly (P < 0.01) larger sniff volume (1.02 ± 0.11 nvu) than a tone which was previously paired with an unpleasant odor during sleep (0.94 ± 0.15 nvu) as shown in figure 3b. A novel association was learned during sleep between a tone and an odor. Their results show that when the tone was presented during sleep without the smell, the participant elicited the same sniff behavior as the normal sniff response would be to that pleasant or unpleasant odor (strong or weak sniff respectively). These results indicate that humans are able to be trace conditioned during sleep.

In a double-blind within-subject crossover design study done by the group of Rasch and colleagues (2007; supporting material: Björn Rasch, Büchel, Gais, & Born, 2007) participants performed a 2-D object-location memory task prior to sleep. In the experimental group the odor of a rose, which is purely olfactory, was presented during this memory task. In a control condition odorless vehicle was delivered during this memory task. Either the odorless vehicle or purely olfactory stimulus was again presented during slow wave sleep. There were 2 other control experiments. One in which the odor was presented during learning and again during REM sleep instead of slow wave sleep, this to see if odor cueing during REM sleep had any effect. Another control experiment consisted of presenting the odor during learning and during a 1 hour interval starting 45 minutes after learning during wakefulness. This control experiment was used to test whether the re-exposure had the same effect during wakefulness compared to sleep. This study also used a nasal mask and used odor as a context cue for the 2-D object-location task. Their olfactometer

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was designed after Lorig (Lorig, 2000) with a constant airflow of 3 l/min. The odor stimulation here was done through a tube in the nasal mask. This 1-m tube with a 12,6 ml volume connected the glass bottles containing the stimulus fluids with the mask, thus allowing rapid odor on- and offset times of 300-500 ms. The experimental odor ("rose") was phenylethylalcohol (99%) diluted in 1,2- propanediol at a concentration of 1:100. Performance on retrieval of the memory task was measured in the morning. Performance was significantly better (P = 0.001) when the odor was presented instead of the vehicle during sleep. Subjects remembered 97.2 ± 5.1% of the card Paris they had learned before sleep after the night the odor was given during sleep. After the vehicle night, subjects remembered 85.8 ± 3.8%. So cued learning with an odor and the presentation of the same odor during sleep enhances declarative memory consolidations. Whether cued learning with a non-hippocampal related task also showed this increase in performance was also tested. A finger sequence snapping task, adopted from previous studies (Walker, Brakefield, Allan Hobson, & Stickgold, 2003; Walker, Brakefield, Morgan, Hobson, & Stickgold, 2002) which did not use hippocampal function was used. Cued learning with this non-hippocampal related task showed no significant (P > 0.2) increase in performance at the retrieval stage after presentation with the odor during slow wave sleep. Results of a fMRI scan showed that the hippocampus is activated when the odor is presented during slow wave sleep (figure 4). This activation is even stronger than odor presentation during wake (Poellinger et al., 2001; Zelano & Sobel, 2005).

Prehn-Kristensen and colleagues (2015) tested the effects of sleep on odor memory in children and adults in a single-blind pseudorandomized controlled trial. There were 2 groups of participants, 30 children and 30 adults. Both the children and the adults were split into 2 groups: the sleep group and the awake group. These 2 groups were instructed to do an encoding task and a retrieval task. In the encoding task 10 odor patches were presented and subjects were instructed to smell the odors and describe them along the different rating scales that were given. 20 different odors could be smelled via patches. During encoding of the odors 10 were used, during retrieval some new odors, distractors, out of the pool of 20 were used as well. For the retrieval task subjects were instructed to smell a range of different target or distractor odors and tell if they remembered the odors from the encoding. For the sleep group the encoding task happened in the evening (20:00 h for children and 21:00 h for adults) In the next morning (8:00 h for children and 9:00 h for adults) the retrieval task was given. The wake group had the encoding task in the morning and the retrieval task in the evening at the same times as the sleep group. They found that adults recognized odors significantly better in the sleep group than the wake group (sleep group: 1.6 ± 0.14 vs wake group: 1.1 ± 0.15; P = 0.38; values are odor recognition accuracy (d’), Mean + SEM). So sleep consolidates memory in adults. There was an opposite effect for children, the wake group scored significantly better in the retrieval task than the sleep group (sleep: 0.8 ± 0.16 vs wake: 1.4 ± 0.12; P = 0.008). Within the sleep group, there is a significant difference between adults and children (P = 0.001). These results are shown in figure 5. An explanation for the difference between children and adults could be that children don’t have the same experiences with odors as adults and their palate is not as refined. Since at the age of 8 not all olfactory functions are defined yet (Chalouhi, 2005; Hugh et al., 2015). So they have a harder timer recognizing the odors compared to adults. This pre-experience might be important when considering associative learning with olfactory stimulation.

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Figure 3 The sniff response revealed learning during sleep. (a) The averaged normalized sniff trace and (inset) sniff volume during sleep following a pleasant (blue) or unpleasant (brown) odor (n = 28). (b) The averaged normalized sniff volume during sleep following a tone (alone) previously paired during sleep with a pleasant odor (blue outline) and a tone (alone) previously paired during sleep with an unpleasant odor (brown outline) (n = 20). Statistical analysis was conducted using two-tailed t test. * P < 0,01, ** P < 0,005. Y-axis units are normalized volume units (nvu). Error bars represent s.e.m. (Adjusted from: Arzi et al., 2012)

Figure 4 Brain activation in response to odor presentation during SWS (threshold set at P < 0.005 uncorrected; superimposed on the average structural MRI of all volunteers). BOLD responses to odor-on periods indicate activation in the left anterior hippocampus (left panels) and in the left posterior hippocampus (right panels). (Adjusted from: Rasch et al., 2007)

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Figure 5 Odor recognition accuracy (d´) of the adult (black) and children (white) sleep and wake group; M, mean; SEM, standard error of means, *, p < .05; **, p < .01; ***, p = .001. (Prehn-Kristensen et al., 2015)

Table 3 Results of associative learning with olfactory stimulation during sleep.

Author and year Blinding / study design

Study objective Primary findings

Arzi et al., 2012 Single-blind randomized controlled trial

Test learning in humans during sleep by pairing pleasant and unpleasant odors with different tones.

Sniff volume was greater following pleasant odors compared to unpleasant odors during sleep. So a novel association was learned between an odor and a tone during sleep.

Rasch et al., 2007 Double-blind within-subjects crossover design

Test memory consolidation during sleep by presenting the odor of a rose or a vehicle during a 2-D object-location memory task

A significant increase in performance on the 2-D object-location memory task was found in the morning where the cued odor was presented during sleep. A significantly stronger activation of the hippocampus during slow wave sleep was found when the cued odor was presented.

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Prehn-Kristensen et al., 2015

Single-blind pseudorandomized controlled trial

Test differences between adults and children in the improvement of odor memory during sleep by the use of an encoding task and a retrieval task

Odor recognition in adults was significantly better in the sleep group compared to the wake group. In children odor recognition was significantly worse than adults in the sleep group, but children’s performance was comparable to adults in the wake group.

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Discussion The neurobiological effects of odors and air quality on sleep are widespread. Due to the personalized repertoire of olfactory receptors a person could smell some odors differently than others, or might have a different preference for a certain odor. Objectively measured sleep quality increases when air quality is good. When outdoor air is clean then a good ventilation system that lets air from outdoor inside should be sufficient to improve the sleep quality. In bigger cities opening a window to improve indoor air quality might not have a beneficial effect. In these cities the outdoor air is usually more polluted and there is usually more noise from outside. These factors can reduce the beneficial effect of a better indoor air quality. To improve sleep quality in bigger cites a proper filter can be installed. (Strøm-Tejsen et al., 2016)

Studies that have tried to find an effect of olfactory stimulation on sleep quality show that odors which are purely olfactory or mildly trigeminal don’t have a negative effect on arousals and awakenings, the trend is more towards a reduction of arousals and awakenings. The effects of “pleasant” odors like jasmine are small but consistent: there is an increase of subjective vigor in the morning just after wake and an increase in slow wave sleep. Although subjective vigor is higher in the morning further research has to be done on how long this increase in vigor lasts in the morning and if vigor is also increased with the use of other purely olfactory odors. There could be a time-of-night effect, which is that olfactory receptors are more susceptible early in the night than later in the night, more specifically during the first NREM-REM sleep cycle. In the study from Goel et al. (2005) lavender oil was used and it did not have a significant effect on sleep efficiency. However lavender still could have an effect on sleep efficiency since these are all healthy subjects, that didn’t have any trouble falling asleep approximately 15 minutes after lights off. This means that their baseline sleep efficiency is high and the effect of lavender had to be bigger for it to have a significant effect on sleep efficiency. The same goes for sleep latency , sleep efficiency and sleep latency are important factors in evaluating objective sleep quality. Learning can occur during sleep, learning of a declarative link between a conditioned stimulus and an unconditioned stimulus is possible in humans even without being conscious about it. A new association between an odor and a tone can already be made during sleep. Perhaps other novel association could be made during sleep, which would be greatly beneficial for humans. While asleep the connections between the different regions in the brain that play a role in olfactory perception get stronger. When an odor is presented during slow wave sleep, after that odor has been presented as context in an object-location memory task, the hippocampus is activated even stronger than during wakefulness. Memory consolidation is improved after re-exposure. Further research much point out whether this is also true for more non-hippocampus related tasks. Pre-experience of an odor might be important as well to make an association during sleep, since children who have less experience with odors than adults recognize odors worse than adults after sleep. It has been shown that an artificial morning dawn simulation light improves subjective well-being, mood, and cognitive performance (Gabel et al., 2013). Artificial dawn simulation light also significantly reduces sleep inertia complaints (Giménez et al., 2010). Beneficial effects of morning light exposure has been shown well into the day. For olfactory stimulation an increase in subjective vigor was found in the morning it would be interesting to research whether olfactory stimulation can also lead to other beneficial effects after sleep like a reduction in sleep inertia complaints for example. Although effects of pleasant purely olfactory stimulation during sleep are mostly positive, negative effects can still occur. A study done by Okabe et al., (2018) has shown that when presenting a favorite odor, in this case phenyl ethyl alcohol (rose-like smell) during REM sleep, it induces a negative dream emotion. These results could be contributed to the enhanced sensitivity of the olfactory epithelia, which occurs when an odor is repeatedly presented during a person’s life like a favorite odor. During REM sleep the amygdala, which mainly processes negative emotions such as fear (Adolphs, Tranel, Damasio, & Damasio, 1994), is activated and the prefrontal cortex, which processes and evaluates olfactory information, such as positive/negative valence (Rolls, Kringelbach,

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& de Araujo, 2003; Royet, Plailly, Delon-Martin, Kareken, & Segebarth, 2003), is deactivated. So individual difference in olfactory stimulation needs to be taken into account in further research. All in all, the neurobiological effects of odors during sleep are that purely olfactory odors increases vigor in the morning as well as slow wave sleep. Olfactory stimulation could improve objective sleep quality by increasing sleep efficiency. Odors can also be used to make novel associations and improve memory consolidation during sleep. The neurobiological effects of air quality during sleep are that subjective sleep quality increases when the indoor air quality is good. The effective usage of a good ventilation system can help with good indoor air quality. Sleep is thought to be important for learning and memory, the roles that odors and air quality play on learning, memory and sleep quality remains an interesting topic where more research can be done.

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