Sleep medicine is a relatively new scientific specialty. Sleep is an important topic in Islamic literature, and the Quran and Hadith discuss types of sleep, the importance of sleep, and good sleep practices. Islam considers sleep as one of the signs of the greatness of Allνh (God) and encourages followers to explore this important sign. The Quran describes different types of sleep, and these correspond with sleep stages identified by modern science. The Quran discusses the beneficial effects of sleep and emphasizes the importance of maintaining a pattern of light and darkness. A mid-day nap is an important practice for Muslims, and the Prophet Muhammad peace be upon him (pbuh) promoted naps as beneficial. In accordance with the practice and instructions of Muhammad (pbuh), Muslims have certain sleep habits and these sleep habits correspond to some of the sleep hygiene rules identified by modern science. Details during sleep include sleep position, like encouraging sleep on the right side and discouraging sleep in the prone position. Dream interpretation is an established science in the Islamic literature and Islamic scholars have made significant contributions to theories of dream interpretation. We suggest that sleep scientists examine religious literature in general and Islamic literature in particular, to understand the views, behaviors, and practices of ancient people about the sleep and sleep disorders. Such studies may help to answer some unresolved questions in sleep science or lead to new areas of inquiry.
Sleep medicine is considered a relatively new field of medicine, but mankind has long been interested in sleep, and culture and religion influence attitudes and beliefs about sleep. In particular, religious literature has many references to sleep.[1,2] Islam emerged as a religion in the seventh century when the Prophet Muhammad peace be upon him (pbuh) started receiving revelations from Allāh (God), known as the Holy Quran (610 C.E.). Most Muslims, with a worldwide population of about 1.6 billion, view Islam as a way of life and follow the instructions of Islam in all of their daily practices, including sleep.[3] The two sources of Islamic jurisprudence are the Quran and Hadith (Sunnah). Muslims believe that Allāh revealed the Quran to the Prophet Muhammad (pbuh) through the angel Gabriel from 610 to 632 C.E., the year of the Prophet's death [verse 17. 106]. The text of the Quran contains 114 chapters (Sūra). Hadith are a collection of narrations concerning the words and deeds of the Prophet Muhammad (pbuh). These were evaluated and gathered into a large collection, mostly during the eighth and ninth centuries.
Islam has great interest in sleep, and sleep is considered as one of the signs of the greatness of Allāh. Sleep is mentioned frequently in the Quran. For example, a well-known verse says, “And among his signs is your sleep by night and by day and your seeking of His bounty, verily in that are Signs for those who hearken” [verse 30.23]. Islam has clear instructions and guidance for followers about the nature of good sleep. In addition, Muslims have shown great interest in dreams and dream interpretation.
In this article, for citation of the Quran, we refer to chapter (Sūra) and verse (Sūra.verse); for citation of Hadith, we refer to the book and Hadith number. For the Quran, we used an English translation that was approved by the Ministry of Islamic Affairs in Saudi Arabia and the Islamic University in Madinah; for Hadith, we cite major books approved by the Ministry of Islamic affairs in Saudi Arabia.[5–7]
In this article, we discuss the Islamic views of sleep based on the Quran and Hadith and the impact of these views on sleep and sleep habits of modern Muslims.
Types of Sleep in the Quran
The Quran frequently mentions sleep. There is a general Arabic word for “sleep” (Noum) and other Arabic words for specific types of sleep. The word “sleep” and its derivatives appear nine times in the Quran. In addition, different Arabic words are used to describe sleep in the Quran, and these may correspond to the different sleep stages identified by modern sleep science:
“Sinah”: This word has been defined as “slumber” or “dozing off for a very short period”, during which there is prompt arousal following environmental stimulation. This may correspond to stage 1 sleep identified by modern sleep scientists. A verse in the Quran uses the word “Sinah” when describing Allāh “No slumber (Sinah) can seize Him nor sleep” [verse 2.255]. In the Quran, sleep implies a manifestation of weakness and bodily need for rest. Therefore, while the Creator (Allāh) does not sleep or doze off, His creations, including mankind, need sleep every day.
“Nu’ass”: Two verses in the Quran use the word “Nu’ass”. One verse says “Remember when He covered you with a slumber (Nu’ass) as a security from him” [verse 8.11]. This describes the fear and stress of the believers during the battle of Badr, when slumber (Nu’ass) provided them with a feeling of security and relief from stress. Nu’ass in this verse implies a short nap, which may correspond to stage 1 and stage 2 sleep identified by modern sleep scientists. It was recently suggested that a short nap can reduce stress and blood pressure (BP), with the main changes in BP occurring between the time of lights off and the onset of stage 1.[8–10] A second verse of the Quran says “Then after the distress, He sent down security upon you. Slumber (Nu’ass) overtook a party of you, while, another party was thinking about themselves (as how to save their own selves)” [verse 3.154].
“Ruqood”: This word has been given several interpretations. In our view, the most appropriate definition is “sleep for a long period”, as Allāh has described the People of the Cave with this term in the Quran[2] “And you would have thought them awake, whereas they were asleep (Ruqood)” [verse 18. 18]. The Quran states that the People of the Cave stayed in their caves for 300 solar years, adding nine (for lunar years) [verse 18. 25], as discussed later
“Hojoo”: This term describes pious believers who fear Allāh, “They used to sleep but little by night (Hojoo). And in the hours before dawn, they were (found) asking (Allāh) for forgiveness” [verse 51. 17-18]. This word indicates “sleep at night”.
“Subaat”: The word “Subaat” is derived from the Arabic word “Sabt”, which means disconnecting.[2] “Subaat” may indicate a disconnection from the surrounding environment during sleep. A verse in the Quran says, “And we made your sleep (Subaat) as a thing for rest” [verse 78.9]. Therefore, “Subaat” may be considered to be “deep sleep”, corresponding to the slow wave sleep identified by modern sleep scientists.
Based on the above, we suggest that the arrangement of sleep stages/states is Sinah and Nu’ass, followed by Hojoo, and Ruqood and then Subaat.
Importance of Sleep
Modern sleep scientists believe that sleep deprivation has deleterious effects on mental concentration, memory, mood, and quality of life. In addition, recent data indicate that sleep deprivation impairs endocrine and metabolic functions.[12,13] Islam also emphasizes the importance of getting enough sleep. One Hadith by the Prophet (pbuh) in Sahih Al-Bukhari (SB) says, “If anyone of you feels drowsy while praying he should go to bed (sleep) till his slumber is over” (SB 210). The Prophet (pbuh) told one of his companions (Ibn Amr) who was praying the whole night “Offer prayers and also sleep at night, as your body has a right on you” (SB 1874). Once the Prophet (pbuh) entered the Mosque and saw a rope hanging in between its two pillars. He said, “What is this rope?” The people said, “This rope is for Zainab, who, when she feels tired, holds it (to keep standing for the prayer.)” The Prophet (pbuh) said, “Don’t use it. Remove the rope. You should pray as long as you feel active, and when you get tired, sleep” (SB 1099). Another Hadith narrated by Aisha (wife of the Prophet [pbuh]) in Musnad Ahmed (MA) tells of a woman from the tribe of Bani Asad, who was sitting with Aisha when Allāh's Apostle (pbuh) came to my house and said, “Who is this?” Aisha replied, “She is so and so”. She does not sleep at night because she is engaged in prayer. The Prophet said disapprovingly, “Do (good) deeds which are within your capacity as Allāh never gets tired of giving rewards till you get tired of doing good deeds” [MA 25244].
Sleep manners
There are numerous Muslim sleep traditions that Muslims try to follow in order to be in accordance with the practice of the Prophet (pbuh) (Sunnah).
Early bedtime and early wake up time
Muhammad (pbuh) encouraged his companions not to be involved in any activity after Isha prayer (darkness prayer, which is around 1.5-2 hours after sunset). The Prophet (pbuh) said, “One should not sleep before the night prayer, nor have discussions after it” [SB 574]. Additionally, Muslims are required to wake up for Fajr prayer, which is about one hour before sunrise. The Prophet did not sleep after Fajr prayer.[2] In addition, the Prophet (pbuh) told his companions that early morning work is blessed by Allāh.
Perform ablution (wudoo) before going to bed and supplicate
It is reported in Sahih Muslim (SM) that one of the companions said that the Prophet (pbuh) told him, “Whenever you go to bed, perform ablution like that for the prayer, and lie on your right side” [SM 2710]. And then he asked him to say the night prayers before sleep.
Dusting and cleaning the bed before sleeping
It has been reported that the Prophet (pbuh) said, “When any one of you goes to bed, he should take hold of the hem of his lower garment and then should clean (his bed) with the help of that and then should recite the name of Allāh” [SM 271].
Sleep position
In Islamic culture, some sleep positions are encouraged while others are discouraged based on the practice (Sunnah) and recommendations of the Prophet Muhammad (pbuh). Thus, many Muslims sleep on their right side, particularly, in the initial part of sleep. Muhammad (pbuh) said, “Whenever you go to bed, perform ablution like that for the prayer, and lie on your right side” [SM 2710). In description of the sleep of the Prophet (pbuh), a Hadith states, “When the Prophet (pbuh) wants to go to sleep, he puts his right hand under his cheek” [SM 2713]. Modern scientific studies have suggested a beneficial effect of right lateral decubitus position on the heart. In particular, one study assessed the autonomic effect of three sleep positions (supine, left lateral decubitus, and right lateral decubitus) in healthy subjects using spectral heart rate variability analysis.[14] The results indicated that cardiac vagal activity was greatest when subjects were in the right lateral decubitus position. In addition, an animal study indicated that vagal stimulation has an antiarrhythmic effect.[15] Several studies have demonstrated that the recumbent position affects autonomic nervous system activity in patients with congestive heart failure, and that there is attenuation of the sympathetic tone when subjects are in the right lateral decubitus position.[16–18] Muslims tend to dislike sleeping in the prone position, and this is discouraged in the Islamic literature, even for infants. The Prophet (pbuh) told a man who was lying on his stomach, “Allāh and his Prophet dislike this position” [Sunan Al-Tirmdhi 2768]. Modern medical studies have concluded that infants who sleep in the prone position have a seven-fold increased risk of sudden infant death syndrome (SIDS). This has led to “back to sleep” campaigns in Britain (1991) and in the United States (1994).[19]
Turning off light before sleep
It is narrated that the Prophet (pbuh) said, “Put out lamps when you go to bed, shut the doors, and cover water and food containers” [SB 5301]. This may correspond with current scientific understanding that it is important to maintain a dark environment during sleep so as not to disrupt the circadian rhythm.
Yawning
Yawning is an unacceptable behavior for Muslims, especially in public places. If yawning occurs, the yawner is instructed to cover his mouth with his hand. The Prophet said, “Yawning is from Satan. If you are about to yawn, you should try to stop it as much as possible. If you yawn, Satan will laugh” [SB 3115].
Naps (Siestas)
Napping is a cross-cultural practice, and modern sleep scientists believe that napping provides benefits for all ages.[20] A short mid-day nap (called Qailulah in Islamic culture) is a deeply embedded practice in the Muslim culture, and it takes a religious dimension (Sunnah) for some Muslims. The Prophet Muhammad (pbuh) said, “Take a short nap, for Devils do not take naps” [Sahih Aljamie. Alalbani 1647]. Another Hadith by Muhammad (pbuh) provided details about the timing of the nap, “Sleeping early in the day betrays ignorance, in the middle of the day is right, and at the end of the day is stupid.” (Fath Al-Bari, p.73). A third Hadith reported in Sahih Al-Bukhari (SB) says, “We used to offer the Jumua (Friday) prayer with the Prophet and then take the afternoon nap” [SB 5923]. Friday is the weekend for Muslims, so napping on Friday may compensate for sleep debt that has accumulated during weekdays.
Previous research has shown that short daytime naps improve vigilance and cognitive functions, and are beneficial for memory consolidation.[21] In particular, a nap as short as 10 min can improve alertness and performance for 2.5-4 hours.[21] A recent study assessed the health effects of napping in 23,681 healthy Greek adults for an average of about six years. After controlling for potential confounders, the researchers concluded that those who napped at least three times weekly for about half an hour had 37% lower coronary mortality than those who did not nap.
Circadian Rhythm
The Quran frequently presents “day” and “night” as significant signs of the creator (Allāh). The Quran mentions the alternation of day and night in 37 places and in many places asks Muslims to observe the succession of night and day. For example, “And it is He who has made the night and the day in succession for whoever desires to remember or desires gratitude” [verse 25.62]. In the Quran, the word “night” always precedes the word “day”, “And We have appointed the night and the day as two signs. Then We have obliterated the sign of the night with darkness, while We made the sign of the day illuminating” [verse: 17.12]. It is clear that the Quran considers humans to be diurnal creatures who need light in the daytime and darkness at night, “And it is He Who makes the night a covering for you, and the sleep a repose, and makes the day Nushūr (i.e., getting up and going about here and there for daily work, after one's sleep at night)” [verse: 25.47]. The Quran stresses the importance of the daily pattern of light and darkness and considers the cycle of night and day as a mercy from Allāh, “Say: See ye? If Allāh were to make the Night perpetual over you to the Day of Judgment, what God is there other than Allāh, who can give you enlightenment? Will ye not then hearken? Say: See ye? If Allāh were to make the Day perpetual over you to the Day of Judgment, what God is there other than Allāh, who can give you a Night in which ye can rest? Will ye not then see? It is out of His Mercy that He has made for you Night and Day, - that ye may rest therein, and that ye may seek of His Grace - and in order that ye may be grateful” [verses 28.71-73].
Muslims have five obligatory prayers per day. The first prayer (Fajr) is at dawn (about one hour before sunrise), so Muslims are obliged to wake up early on weekdays and weekends; the last prayer (Isha) is in the evening, about 1.5-2 hours after sunset. Summer nights have earlier dawn and shorter nights, so Muslims may have less night sleep during the summer. Sleep scientists have not yet studied the physiological effects of this, although available evidence suggests a possible seasonality effect in bed times and wake times.[22,23] Honma et al. studied 10 healthy male volunteers from Japan and reported that wake-up time in the summer was 60 min earlier than in the winter and that bedtime was earlier in summer, resulting in a slightly longer total time in bed during the winter than summer.[24] They also reported that the acrophase (circadian maximum) for core body temperature and plasma melatonin changed with the seasons, with a 2 hour phase delay in winter, compared to summer.[24] Another study examined nine healthy males at the Antarctic zone for 15 months. The peak phase of melatonin rhythm was phase delayed by 4.1 hours in winter, compared to summer. In addition, the trough phase of rectal temperature rhythm in two of three subjects was phase delayed by approximately 2 hours in winter. However, in this study there was no change in total sleep time in winter, compared to summer.[23] Seasonal changes in the phase of circadian rhythms are normally due to seasonal changes in the intensity of light and in the times of sunset and sunrise.
Unique topics about sleep in Islamic culture
In this section, we will discuss sleep and death, the story of the Companions of the Cave, and dreams and dream interpretation in Islamic culture.
Sleep and death
The Quran indicates some resemblance between sleep and death. The Quran uses “Wafat” to describe death, and one of the verses states, “It is Allāh Who takes away the souls (Wafat) at the time of their death, and those that die not during their sleep. He keeps those (souls) for which He has ordained death and sends the rest for a term appointed. Verily, in this are signs for people who think deeply” [verse 39:42]. Another verse states, “It is He Who takes your souls (Wafat) by night (when you are asleep), and has knowledge of all that you have done by day, then He raises (wakes) you up again that a term appointed (your life period) be fulfilled, then (in the end) to Him will be your return. Then He will inform you of that which you used to do” [verse 6.60]. The Quran divides the “state of self” into two parts, a state of consciousness and a state of “Wafat”.[25] The state of “Wafat” is divided into sleep (temporary death) and death (real death). Thus, the above verses indicate that Allāh takes and retains souls during real death, but releases souls after rest for an appointed term. The Hadith from the Prophet Muhammad (pbuh) supports this view. It is reported that whenever the Prophet (pbuh) went to bed, he said, “O’ Allāh, it is with Thine Name that I live and it is with Thine Name that I die”, and when he awoke, he said, “Praise is due to Allāh, Who gave us life after our death (sleep) and unto Thee is resurrection” [SM 2711]. Muslims believe that people in Heaven do not sleep, because sleep is a form of death. The Prophet (pbuh) was asked, “Do people of Heaven sleep?” He answered: “Sleep is the brother of death. People of Heaven do not sleep”
The people (companions) of the cave
In Sūra Al-Kahf (the cave), the Quran describes the People or Companions of the Cave (as’hab al-Kahf), known in Christian literature as “the Seven Sleepers of Ephesus”.[27] The verses [18.9-26] describe young believers who found refuge from prosecution in a cave. When the boys asked Allāh for mercy, He put them into a sleep state that lasted for 300 solar years, adding nine (for lunar years). We present this story, because it provides information that corresponds with our current understanding of sleep and the effect of light and noise on sleep.
The verses describe the regular turning of the boys from side to side during their long sleep, “We turned them on their right and on their left sides” [verse 18.18]. Modern science has documented that staying on one side for long periods can cause bed sores. In addition, prolonged immobility increases the risk of numerous conditions, including thrombosis. Therefore, in modern medical practice, patients who are bed-ridden are turned regularly.
The description of the Companions of the Cave portrays their conditions during sleep. One verse states that the boys appeared to those who saw them as if they were awake, when in fact they were asleep, “And you would have thought them awake, whereas they were asleep” [verse18.18]. This verse suggests that their eyes were open and blinking when they were asleep. In the context of modern sleep science, open eyes allow more light perception and are important for the regulation of circadian rhythms.
During their sleep, the Quran describes sunlight that comes with sunrise and leaves at sunset and to maintain the light-dark cycle, “And you might have seen the sun, when it rose, declining to the right from their Cave, and when it set, turning away from them to the left, while they lay in the midst of the Cave. That is one of the signs of Allāh” [verse 18.17]. New discoveries revealed that a regular light-dark cycle is essential for the calibration of circadian rhythms, and maintenance of the circadian pattern of body functions and hormonal secretion.[28] To create a suitable environment for sleep, the Quran mentions that the hearing of the boys was sealed up during their entire sleep period, “Therefore, We covered up their (sense of) hearing (causing them to go in deep sleep) in the Cave for a number of years” [verse 18.11]. Although sensitivity to noise decreases during sleep, modern scientists believe that the sleeping body still responds to noise stimulation.[29] Noise during sleep has a negative impact on the quality of sleep because it increases arousal, increases changes in sleep stages, decreases slow wave sleep, and disturbs the rhythmicity of rapid eye movement (REM) sleep.[29,30] In addition, noise during sleep may disturb the autonomic and endocrine responses of the body. Although autonomic reactions that occur during sleep may be small, their accumulation over time may result in harmful effects, such as increased risk for cardiovascular disease.[31] Subjects do not become adapted to these changes following long exposure times.[29]
Dreams
Detailed discussion of dreams in the Muslim culture is beyond the scope of this article. Instead, we provide an overall summary of the importance of dreams in Muslim culture. Muslims in general have great interest in dreams and dream interpretation. In general, Muslims consider dream to be a kind of supernatural perception. One Hadith states that the Prophet (pbuh) said, “A good dream vision of a pious man is a forty-sixth part of prophecy” [SM 2263]. It has also been reported that the Prophet (pbuh) said, “A good vision (ru’ya) is from Allāh and a bad dream (hulm) is from Satan; so if one of you sees anything (in a dream which he dislikes), he should spit on his left side thrice and seek refuge with Allāh from its evil, and then it will never harm him” [SB 3118).
Oneiromancy is a traditional type of dream interpretation that is common in the Muslim world. In general, Muslims have much higher regard for dreams and dream interpretation than people from Western societies.[4] Muslim countries traditionally used the terms Tabir or Tafsir for “dream interpretation”, and dreams continue to play an important role in the lives of modern Muslims.[32,33] Muslim interest in dreams and dream interpretation has not been well documented in the English literature, and most Western dream researchers are not familiar with the rich traditions of dreams and dream interpretation in Islam.[4] The theories, insights, and observations of dreams proposed by Muslims over the past 1400 years correspond with many of the recent theories developed by Western psychologists during the past 150 years.[4] Traditionally, Muslims believe that dreams appearing in the last third of the night are more truthful. This correlates with the current scientific understanding that the longest periods of REM sleep occurs during the last third of the nocturnal sleep period, when dream imagination is most active.
The Quran uses several terms to refer to dreams, such as ru’ya (vision) (verses 17.60, 37.105, 48.2), hulm (dream) (verses 21.5, 52.3), manam (sleep) [verse 37.10], and bushra (tidings) [verse 10.6]. Because of the central role of the Quran in the Muslim faith, discussions of dreams are fundamental to Islamic dream interpretation. Dream description plays a major role in three Sūras (chapters) of the Quran:
Sūra 12, Yussuf (Joseph): This Sūra provides a condensed version of the story of Joseph and some of the best known references to dream interpretation.
Sūra 37, As-Sāffāt (Ranks): This Sūra focuses on Allāh's command to the Prophet Abraham to sacrifice his son.
Sūra 8, Al-Anfāl (Spoils): This Sūra describes a dream of Prophet Muhammad (pbuh). “(And remember) when Allāh showed them to you as few in your (i.e., the Prophet's) dream; if He had shown them to you as many, you would surely have been discouraged, and you would surely have disputed making a decision. But Allāh saved you” [verse 8.43]. This verse describes the experience of the Prophet (pbuh) the night before a particular battle (Badr), when the Muslim army was across the valley from its enemy.
The night journey (Laylat al-Mi’raj) in Sūra 17 (Al-Isrā) says, “Glory be to Him (Allāh) Who took His slave (Muhammad) for a journey by night from Al-Msajid Al-Harām (in Mecca) to Al-Masjid Al-Aqsā (in Jerusalem) whose surroundings We have blessed, that We might show Him (Muhammad) some of Our signs” [verse 17.1]. Some Western scholars who have written about dreams in the Quran consider this journey as one of the dreams of Muhammad (pbuh).[4,34] However, although this journey occurred in a short period in one night, in the Muslim faith, this miraculous journey is considered to be a physical journey, not a dream. In particular, the body and soul of Muhammad (pbuh) travelled from Mecca to Jerusalem and then ascended to heaven. This led him to the wonders of heaven, where he met with many prophets and messengers who had gathered to meet him, and He led them in prayers. Therefore, we will not discuss this journey as a dream.
Some interpreters of the Quran have interpreted verse 39.42 (“It is Allāh who takes away the souls (Wafat) at the time of their death, and those that die not during their sleep. He keeps those (souls) for which He has ordained death and sends the rest for a term appointed. Verily, in this are signs for people who think deeply”) as Allāh seizing souls during death and sleep (dream). For instance, the Islamic scholar, Al-Qurtubī (1214-1273 C.E.), noted that true dreams are visions experienced while the soul is separated from the body during sleep, whereas nightmares and lying dreams occur when the soul has returned to the body, but before it has again taken firm root.
Numerous Muslim philosophers have proposed theories of dream interpretation. Ibn Arabi (1164-1240 C.E.) proposed a metaphysical system that merged Islamic theology and Greek philosophy.[4,35] Ibn Sirin (653-728 C.E.) is the best-known dream interpreter in Islamic history,[4,36] and his method of dream interpretation reflects the fact that dream interpretation is important in the Quran and Hadith. He proposed that the interpretation of dreams depends on the personal characteristics and life circumstances of the individual.[4] Ibn Khaldūn, a great Muslim scholar and thinker (1332-1402 C.E.), considered dream interpretation to be a science.[37] In the monumental Muqaddimah (An Introduction to History), he classified three types of dreams: (i) dreams from Allāh (Allāh), which are clear and unmistakable in their meaning and content; (ii) dreams from Angels, which are received in the form of allegory and require interpretation; and (iii) dreams from Satan, which are confused dreams that are futile.[2,37]
Conclusion
In the past few decades, there has been a significant increase in our knowledge of sleep physiology, sleep disorders, and the importance of sleep. Islam and other ancient religions also provide significant information about the historical and cultural views of sleep, and these precede modern scientific studies by hundreds or thousands years.[1,2] The Quran describes different types of sleep, and these correspond with different sleep stages identified by modern sleep scientists. About 1400 years ago, Muhammad (pbuh) stressed the importance of sleep for good health and the Quran stresses the importance of the alteration of night and day. A nap (Qailulah) is a well-established cultural practice in the Islamic culture. For some Muslims, the nap has religious dimensions. Modern sleep scientists acknowledge the beneficial effect of short naps. Muslims have been following certain sleep habits for hundreds of years, following the instructions and practices of their Prophet (pbuh). Modern sleep scientists currently recommend many of these same practices. Dream interpretation is an established science in the Muslim literature and Islamic theories of dream interpretation correspond with many theories currently proposed by modern sleep scientists.
We suggest that sleep scientists examine religious literature to understand the views, behaviors, and practices of ancient people in regard to sleep and sleep disorders. Such studies may help to answer unresolved questions in sleep science or lead to new areas of inquiry.
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