This piece was written by one of our contributors; Psychology MSc student – Ellie Wright
We do it every day. It’s free, rarely has negative side effects, and is so important for our whole bodies: our mental and physical health (1, 2, 3). But we are becoming so bad at sleeping that experts and the Centre for Disease Control and Prevention have named the problem a sleep epidemic (4, 5).
Lifestyle behaviours can prevent sleep onset. Our 24/7 world encourages us to do things which disrupt our internal clocks. Examples include:
?over exposure to light after dark (6)
? too little daylight (7)
✈️transcontinental travel (8)
?⚕️shift work (9)
?persistent snacking, including late at night (10, 11)
However, from the moment we open our eyes, we can make changes which protect sleep despite these demands.
Poor sleep (short or disturbed sleep) changes our bodies and behaviour (1). Understanding how can motivate us to prioritise habits which promote sleep onset.
The problem: how much do we sleep?
Not enough! Poor quality sleep is an increasingly common condition and normally presents as partial sleep deprivation day after day (12). How much sleep we need depends on our age. For adults, the recommendation is 7-9 hours (13, 14). Short sleep is growing trend around the world. In 1960 we were getting 8.5 hours sleep each night but now, on average, we are sleeping just under 7 hours, and around a fifth of us get less than 6 hours (15, 16).
Globally, we don’t prioritise sleep
We increasingly undervalue sleep. In Japan, where the average night of sleep is 6 hours, there is even a word for being present whilst asleep ‘inemuri’ (17). And, short sleep is learned from a young age: children’s sleep has declined by 70 minutes since the 1900s (15, 18).
What is sleep?
When we close our eyes, our brain’s activity changes. We cycle through five sleep stages, an average of four to five times a night with each loop taking progressively longer.
Each stage is characterised by electrical activity. Neurons in our brain start firing in synchrony like a Mexican wave, playing out a hormonal symphony (19). Our heart rate, breathing rate and brain activity gradually slow in stages one to four. Stage five: rapid eye movement sleep (REM) resembles wakefulness; our eyes dart behind our eyelids and the sleep stage cycle starts again (2).
How we fall asleep
We have sleep and wake promoting systems. These networks of neurotransmitters communicate brain state changes, working against each other to help and halt sleep (20)
Narcolepsy
Orexin is one of many important neuropeptides. This molecule signals the change in sleep/wake brain states, meaning we can only be in one at any given time (21). Without enough of it people seesaw between wakefulness and sleep; a condition we call narcolepsy (22).
Circadian rhythms: our 24 – 25 hour body clocks
Cells in our brains and bodies follow an internal master clock: our circadian rhythm. With each rotation of the sun around the earth, light intake and feeding behaviour resets our circadian rhythm to Earth’s 24 hour day. This is disagreement about the exact timing of our circadian clocks, but on average they are a bit longer than 24 hours, making daily reset vital (23, 24)
Each cell in our body has a clock (25). Light passes through our retina and into the circadian pacemaker – part of the brain called the suprachiasmatic nucleus or SCN. This brain region coordinates a complex hormonal circuit which sends signals to organs, muscles and tissues, synchronising our body with the time of day and helping promote sleep onset (21).
How well we sleep and eat in synchrony with night and day determines the healthy or disordered production of hormones which change our behaviour. These include hormones responsible for stress, appetite, sleepiness and insulin control (21).
Why we sleep
We spend a third of our lives asleep. Animal studies suggest without it we would die (27). But anything could happen while we sleep! Dr Alan Rechtschaffen summarises this problem: “if sleep doesn’t serve a vital function, it’s the greatest mistake evolution ever made” (28)
The variation of sleep habits in the animal kingdom suggest sleep balances benefits of rest with survival needs (29). Giraffes sleep for just two hours, perhaps because it takes 30 seconds to stand up, dolphin’s sleep with half a brain at time and bats sleep for 20 hours (30).
The benefits of sleep
Current evidence suggests sleep improves how we feel, behave and think and how much people like us (1, 31, 32, 33). It replenishes energy, “boosts” our immune systems, protects physical and mental health and prevents against psychiatric conditions (35 – 42). Poor sleep quality possess risks to mental and physical health. But how?
?♀️Overnight therapy
Sleep plays back memories to remember, emotionally process and strip them of emotional feeling without affecting content (31). Sleep studies show how dreams become progressively less negative throughout the night, preparing our mood for waking (32, 2).
?Connection
Sleep helps us socially connect by improving our mood and opening our body language (1). People who did not know they were judging people involved in a sleep study, liked and trusted the control group who had normal sleep more than participants who had been partially sleep deprived on 4-5 hours for just one week (1).
??Sleep deprivation causes loneliness
This same study found that lack of sleep causes social isolation (1). It makes us hypersensitive so our body language changes and we sit further away. The fact that naive judges liked people who had just one week of partial like deprivation less than those who had slept 7-9 hours matters for our physical and mental health. Loneliness poses a risk to our physical health greater than tobacco or obesity (33).
?Happiness
Sleep helps sustain happiness. People tested on 7 hours sleep had good moods but when the same group of participants were sleep deprived, on just under five hours a night, their moods become low, halving on the happiness scale, and their emotional and physical complaints increased four fold (66). Their mental states returned to normal when they returned to sleeping seven hours a night.
? Memory consolidation
Sleep helps us to remember and store and new memories (33). Avoiding disruption in the first half of sleep, when slow wave sleep dominates, facilitates this process (2).
? Learning? Take a nap!
A nap helps consolidate learning, moving learnings from short-term memory in the hippocampus to long term memory in the neocortex. Brain scans show this relocation with lists of new words (34).
?Problem solving
Our problem solving ability increases on difficult tasks after sleep. Rest activates the brain’s Default Mode Network which uses unconscious processing to find hidden rules (35, 36).
⏩ Maximising brain capacity
Forgetting is an important part of sleep. Our brains prioritise what we need to remember and forget what we don’t for sustainable and effective brain productivity (37).
??Strength
Hormonal activity during sleep is vital for our physical health, particularly growth and repair. 60- 70% of growth hormones are released during first few hours of sleep timing. Without sleep, very little amounts are released (38).
???Energy restoration
Sleep acts as a power saving mode, reducing calorie use at a time when, if awake, darkness would make us inefficient (39). It helps us replenish brain energy stores; blood glucose consumption is twice as high when we are awake (40).
❤️ Sleep helps to protect against heart disease, cancer and stroke
Evidence suggests people who sleep 7-8 hours have lower rates of cardiovascular disease, cancer and stroke. Their immune systems have adequate sleep for healthy levels of growth and stress hormones, helping to maintain immunological memory which is key for fighting illness and disease (41, 42).
? Mood instability
REM sleep is really important for our psychological health. Just a week without enough of it, or if disturbed, can lead to memory problems and a lack of impulse control; we can become moody, irritable and aggressive (44).
? ? Decreased ability to pay attention and think clearly
Lack of sleep significantly reduces focus and our ability to process what’s happening around us (45). For drivers, this really matters. In experimental conditions, sleep restricted drivers, who didn’t feel tired, crossed motorway lines significantly more than well slept drivers (46). Sleep deprivation makes you 2.6 times more likely to fall asleep at the wheel (47).
?⚕️ Decline in accuracy, speed and understanding
Shift work impacts workers more than we imagine. Despite care workers thinking they adapted to night shifts, when assessed for accuracy, speed and understanding their thinking performance was still just as impaired as the first night of their rotation (48).
? Increased sensitivity to pain
If we don’t get enough slow wave sleep (stages 1-4), we are more likely to suffer from aches, pains and headaches (49).
? Insomnia & Depression: a co-occurrence
Insomnia may cause depression and depression may cause insomnia; the direction is unclear. But, evidence suggests depression is 10 times more likely to occur in insomniacs (50).
? Sleep disturbance can predict psychiatric disorders
Sleep is linked to most psychiatric disorders and can emerge years before a diagnosis such as bipolar disorder (51).
?? Eating all the time
Insufficient sleep makes us hungrier. It changes the neuroendocrine system which controls appetite. In a lab experiments, people who are sleep deprived tend to keep meal portions the same but snack late into the night eating around 130% of their daily intake with around 500 of these calories consumed late at night (10, 52) . Outside the lab, an app based experiment where dieticians analysed food photos, confirmed this sleep-snacking link (53).
? What we want to eat changes.
Sleep deprivation changes food preference. Brain scans show increased activity in reward pathways even when well nourished, making high energy food more appealing (54).
?? Obesity
Obesity and sleep exist in a vicious cycle. Staying up late increases calorie consumption but doesn’t use much energy – around 17 kcal/ hour or half a biscuit over three hours (10, 55). Short sleepers were found to have a 45% increased chance of developing obesity which in turn disrupts sleep (10, 52, 56).
???? Who you are can change risk of weight gain
Short sleep may make men more prone to gaining more weight than women and people of African American heritage may be more likely to gain weight than Caucasians. This was discovered following the same sleep restriction of four hours sleep for five nights (52) And, people of Black and Hispanic origin are more likely than people of Caucasian heritage to get less than 6 hours sleep a night, increasing health risks (57)
❤️ Sleep deprivation is associated with poor heart health
Sleeping too much or too little is associated with heart disease and stroke. Increased activity in the nervous system may be the mechanism by which this physiological change occurs as it is associated with diabetes, heart disease and hypertension (42).
?Diabetes Type II
Poor sleep quality poses a diabetes type II risk of a similar scale to obesity, family history and low physical activity (58). Short sleep, difficulty falling asleep and difficulty staying asleep increase diabetes type II risk by 28%, 57% and 84% respectively(59). This may be because these sleep disorders are associated with decrease of insulin sensitivity without compensation from a sleep dependent beta cell.
Diabetes may be treated in future by prolonging slow wave sleep. We know this because reducing healthy people’s slow wave sleep for just three nights reduced insulin sensitivity by 25%, causing reduced glucose tolerance(60).
?Poor sleep quality may speed up ageing of gene protectors
In humans, people with better sleep quality show less ageing in their telomere length compared to those with less sleep (61). Telomeres protect molecules from deterioration.
So how can we engineer better sleep?
We can engineer sleep habits from the moment we wake up. These can teach the body when it’s bedtime and protect the quality of the sleep we have.
? Routine
Sleep hygiene uses our reliance on habit to build routines which train our circadian rhythms for sleep. Surrounding ourselves with ‘zeitgebers’ – environmental cues like books in the bedroom facilitates sleep onset (62). Routine functions by conditioning part of our brain called the suprachiasmatic nucleus (SNC) to prepare for and induce sleep.
☀️Get enough daylight
Exposing ourselves to natural light in the day, for example walking on a lunch break, helps sleep onset at night. Melatonin release works like an elastic band; the more we suppress it by getting outside in the day and evening, the more it rises in the dark to promote sleep (7).
?Avoid light after dark
Overexposure to light, in particular electric lighting, late at night disrupts our sleep promoting melatonin levels meaning we feel less tired, particularly so for women!(63)
? Avoid blue light for the hour before bed
Blue and high intensity light before bed delays sleep onset and disrupts quality of sleep, making it less efficient and less able to process glucose (64). LED lights, tablets and smart phones have this blue high energy light.
? LCD screens use different light so can help ruminators
LCD screens have a flatter light so can help people who struggle from anxiety and struggle to switch off.
? Eat early to extend the overnight fast for a higher metabolism
Try to avoid late night snacking to improve sleep onset and boost your metabolism (53).
⛔️ Resist staying up late
As 24 – 25 hour humans in a 24 hour world, we tend to want to stay up a bit later each night and wake up later. Using natural light helps to keep us in synch with planet Earth (23, 24).
? Insomnia
The hypothalamus is part of the brain home to sleep promoting cells; and GABA is the lead neurotransmitter which can turn off wake promoting cells. Insomnia medication includes GABAa agnoinists to encourage the onset of sleep(65).
?⚕️Shift work & Transcontinental travel
We can minimise shift work disruption by engineering our light environments. Sleeping in the evening before a night shift and using bright lights at work suppresses sleep hormone melatonin (9).
Conclusion
Sleep is so important for sustainable health and wellbeing. Poor sleep can cause neural biological changes which impact the health of our whole bodies – mental and physical. Acknowledging the importance of sleep and what might be preventing us from getting enough of it can foster practical rest protective habits that work for us.
If you are struggling to fall asleep, talk to your doctor. Worrying about sleep onset can make it harder to fall asleep and addressing it is important to help feel more like you!
References
(1) Simon E & M Walker (2018). Sleep loss causes social withdrawal and loneliness. Nature 9:3146. https://www.nature.com/articles/s41467-018-05377-0
(2) Diekelmann S & J Born (2010) The memory function of sleep. Nature Reviews Neuroscience 11(2):114-26 https://www.ncbi.nlm.nih.gov/pubmed/20046194
(3) Besedovsky, L., Lange, T., & Born, J. (2011). Sleep and immune function. Pflugers Archiv : European journal of physiology, 463(1), 121–137. doi:10.1007/s00424-011-1044-0 https://www.ncbi.nlm.nih.gov/pubmed/22071480
(4) Centre for Disease Control and Prevention. Insufficient sleep is a public health problem. CDC; 2015.https://www.cdc.gov/features/dssleep. [Google Scholar]
(5) Krause, A. J et al (2017). The sleep-deprived human brain. Nature reviews. Neuroscience, 18(7), 404–418. Journal of Pineal Research doi:10.1038/nrn.2017.55
(6) Santhi et al (2013) The spectral composition of evening light and individual differences in the suppression of melatonin and delay of sleep in humans. https://www.ncbi.nlm.nih.gov/pubmed/22017511
(7) Wright, K et al (2013). Entrainment of the human circadian clock to the natural light-dark cycle. Current biology : CB, 23(16), 1554–1558. doi:10.1016/j.cub.2013.06.039 https://www.ncbi.nlm.nih.gov/pubmed/23910656
(8) Archer et al. (2014) Mistimed sleep disrupts circadian regulation of the human transcriptome. PNAS https://www.pnas.org/content/111/6/E682
(9) Santhi, N et al (2008). The impact of sleep timing and bright light exposure on attentional impairment during night work. Journal of biological rhythms, 23(4), 341–352. doi:10.1177/0748730408319863
(10) Broussard et al (2015). Elevated ghrelin predicts food intake during experimental sleep restriction. Obesity (Silver Spring, Md.), 24(1), 132–138. doi:10.1002/oby.21321 https://www.ncbi.nlm.nih.gov/pubmed/26467988
(11) Dennis et al (2016). Phenotypic stability of energy balance responses to experimental total sleep deprivation and sleep restriction in healthy adults. Nutrients https://www.mdpi.com/2072-6643/8/12/823/htm
(12) McAllister et al (2009). Ten putative contributors to the obesity epidemic. Critical reviews in food science and nutrition, 49(10), 868–913. doi:10.1080/10408390903372599 https://www.ncbi.nlm.nih.gov/pubmed/19960394
(13) Hirshkowitz M (2015) The National Sleep Foundations sleep time duration recommendations: methodology and results summary. Sleep Health (2015) http://dx.doi.org/10.1016/j.sleh.2014.12.010
(14) Watson Net al (2015) . Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep 38(6):843–844.
(15) Youngsted et al (2015). Has adult sleep duration declined over the last 50+ years?. Sleep medicine reviews, 28, 69–85. doi:10.1016/j.smrv.2015.08.004 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4769964/
(16) Paruthi S et al. Recommended amount of sleep for pediatric populations: a consensus statement of the American Academy of Sleep Medicine. J Clin Sleep Med.2016;12(6):785–786.
(17) Williams, N et al (2015). Racial/ethnic disparities in sleep health and health care: importance of the sociocultural context. Sleep health, 1(1), 28–35. doi:10.1016/j.sleh.2014.12.004
(18) Matricciani L et al (2012) In search of lost sleep: secular trends in the sleep time of school-aged children and adolescents https://www.ncbi.nlm.nih.gov/pubmed/21612957
(19) Eagleman, D (2015) The Brain: The Story of You
(20) Saper, & Scamell (2005) Homeostatic, circadian, and emotional regulation of sleep. Nature https://www.ncbi.nlm.nih.gov/pubmed/16251950
(21) Saper et al. (2010) Sleep state switching. Neuron 68(6) 1023-1042 https://doi.org/10.1016/j.neuron.2010.11.032
(22) Okawa M, Uchiyama M (December 2007). “Circadian rhythm sleep disorders: characteristics and entrainment pathology in delayed sleep phase and non-24-h sleep–wake syndrome” (PDF). Sleep Med Rev. 11 (6): 485–96. doi:10.1016/j.smrv.2007.08.001.
(23) Micic G, de Bruyn A, Lovato N et al, The endogenous circadian temperature period length (tau) in delayed sleep phase disorder compared to good sleepers (J Sleep Res. 2013 Dec;22(6):617-24)
(24) Czeisler C et al, Stability, precision, and near-24-hour period of the human circadian pacemaker (Science. 1999 Jun 25;284(5423):2177-81)
(25) Garaulet, M., Corbalán, M. D., Madrid, J. A., Morales, E., Baraza, J. C., Lee, Y. C., & Ordovas, J. M. (2010). CLOCK gene is implicated in weight reduction in obese patients participating in a dietary programme based on the Mediterranean diet. International journal of obesity (2005), 34(3), 516–523. doi:10.1038/ijo.2009.255 https://www.ncbi.nlm.nih.gov/pubmed/20065968
(26) Dijk & Czeisler (1995) Contribution of the circadian pacemaker and the sleep homeostat to sleep propensity, sleep structure, electroencephalographic slow waves, and sleep spindle activity in humans Journal of Neuroscience 1 May 1995, 15 (5) 3526-3538; DOI: 10.1523/JNEUROSCI.15-05-03526.1995
(27) Everson et al. (1989) Sleep deprivation in the rat: III. Total sleep Deprivation. Sleep 12(2): 13-21 https://www.ncbi.nlm.nih.gov/pubmed/2928622
(28) Rechtschaffen, A. (1978) University of Chicago Sleep Laboratory, Smithsonian Institute, November,
(29) Siegel, J. M. (2008). Do all animals sleep?. Trends in neurosciences, 31(4), 208-213
(30) Joiner W. J. (2016). Unraveling the Evolutionary Determinants of Sleep. Current biology : CB, 26(20), R1073–R1087. doi:10.1016/j.cub.2016.08.068 https://www.ncbi.nlm.nih.gov/pubmed/27780049
(31) Walker, M. P., & van der Helm, E. (2009). Overnight therapy? The role of sleep in emotional brain processing. Psychological bulletin, 135(5), 731–748. doi:10.1037/a0016570
(32) Vandekerckhove M & Cluydts R. (2010) The emotional brain and sleep: an intimate relationship. https://www.ncbi.nlm.nih.gov/pubmed/20363166
(33) Born, J., & Wilhelm, I. (2011). System consolidation of memory during sleep. Psychological research, 76(2), 192–203. doi:10.1007/s00426-011-0335-
(34) Davis MH, Di Betta AM, Macdonald MJE, Gaskell MG. Learning and consolidation of novel spoken words. Cogn Neurosci. 2009;21:803–820. [PMC free article] [PubMed] [Google Scholar]
(35) Stickgold R (2005).Sleep-dependent memory consolidation.Nature 437 1272–1278
(36) Sio UN1, Monaghan P, Ormerod T. (2013) Sleep on it, but only if it is difficult: effects of sleep on problem solving. Memory Cognition 1(2):159-66. doi: 10.3758s13421-012-0256-
(37) Tononi G & Cirelli C. (2006) Sleep function and synaptic homeostasis. Sleep Medicine Reviews 10(1) https://doi.org/10.1016/j.smrv.2005.05.002
(38) Spiegel, Leproult & Van Cauter (1999) Impact of sleep debt on metabolic and endocrine function. Lancet 23;354(9188):1435-9. DOI: 10.1016/S0140-6736(99)01376-8
(39) Berger, R & Phillips, N. (1995). Energy conservation and sleep. Behavioural Brain Research, 69(1), 65-73. doi:10.1016/0166-4328(95)00002-
(40) Scharf, M., Naidoo, N., Zimmerman, J., & Pack, A. (2008). The energy hypothesis of sleep revisited. Progress in Neurobiology, 86(3), 264-280. doi:10.1016/j.pneurobio.2008.08.003
(41) Besedovsky, L et al (2011). Sleep and immune function. Pflugers Archiv : European journal of physiology, 463(1), 121–137. doi:10.1007/s00424-011-1044-0
(42) Nagai et al (2010). Sleep duration as a risk factor for cardiovascular disease- a review of the recent literature. Current cardiology reviews, 6(1), 54–61. doi:10.2174/157340310790231635
(43) Holt-Lunstad, J., Smith, T. B., & Layton, J. B. (2010). Social relationships and mortality risk: a meta-analytic review. PLoS medicine, 7(7), e1000316. doi:10.1371/journal.pmed.1000316
(44) Ellman et al. REM deprivation: a review. In: Ellman SL, Antrobus JS, editors. The Mind in Sleep: Psychology and Psychophysiology. John Wiley; NY: 1991. pp. 327–376.
(45) Lim, J., & Dinges, D. F. (2010). A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological bulletin, 136(3), 375–389. doi:10.1037/a0018883
(46) Philip et al (2005) Fatigue, sleep restriction and driving performance. Accident and analysis prevention DOI: 10.1016/j.aap.2004.07.007
(47) Drowsy driving – 19 states and the District of Columbia, 2009-2010.Centers for Disease Control and Prevention (CDC).MMWR Morb Mortal Wkly Rep. 2013 Jan 4; 61(51-52):1033-7. https://www.ncbi.nlm.nih.gov/pubmed/23282860/
(48) Ganesan, S et al. (2019). The impact of shift work on sleep, alertness and performance in healthcare workers. Scientific Reports, 9(1), 1-13. doi:10.1038/s41598-019-40914-x
(49) Lentz et al. (1999) Effects of selective slow wave sleep disruption on musculoskeletal pain and fatigue in middle aged women. Journal of Rheumatology . 26(7):1586-92.
(50) Taylor et al (2005). Epidemiology of insomnia, depression, and anxiety. Sleep, 28(11), 1457-64.
(51) Ritter et al (2011). The role of disturbed sleep in the early recognition of bipolar disorder: A systematic review. Bipolar Disorders, 13(3), 227-237. doi:10.1111/j.1399-5618.2011.00917.x
(52) Spaeth, et al (2013). Effects of experimental sleep restriction on weight gain, caloric intake, and meal timing in healthy adults. Sleep, 36(7), 981–990. doi:10.5665/sleep.2792
(53) Gill, S., & Panda, S. (2015). A smartphone app reveals erratic diurnal eating patterns in humans that can be modulated for health Benefits. Cell metabolism, 22(5), 789–798. doi:10.1016/j.cmet.2015.09.005
(54) Reutrakul, S., & Van Cauter, E. (2018). Sleep influences on obesity, insulin resistance, and risk of type 2 diabetes. Metabolism, 84, 56-66. doi:10.1016/j.metabol.2018.02.010
(55) Jung, C. M., et al (2010). Energy expenditure during sleep, sleep deprivation and sleep following sleep deprivation in adult humans. The Journal of physiology, 589(Pt 1), 235–244. doi:10.1113/jphysiol.2010.197517
(56) Wu, Y. et al (2014). Sleep duration and obesity among adults: A meta-analysis of prospective studies. Sleep Medicine, 15(12), 1456-1462. doi:10.1016/j.sleep.2014.07.018
(57) Sheehan, C., Frochen, S., Ailshire, J., & Walsemann, K. (2019). Are u.s. adults reporting less sleep?: Findings from sleep duration trends in the national health interview survey, 2004-2017. Sleep, 42(2). doi:10.1093/sleep/zsy221
(58) Anothaisintawee et al (2016). Sleep disturbances compared to traditional risk factors for diabetes development: Systematic review and meta-analysis. Sleep Medicine Reviews, 30, 11-24. doi:10.1016/j.smrv.2015.10.002
(59) Cappuccio, F. P., D’Elia, L., Strazzullo, P., & Miller, M. A. (2009). Quantity and quality of sleep and incidence of type 2 diabetes: a systematic review and meta-analysis. Diabetes care, 33(2), 414–420. doi:10.2337/dc09-1124
(60) Tasali, E., Leproult, R., Ehrmann, D. A., & Van Cauter, E. (2008). Slow-wave sleep and the risk of type 2 diabetes in humans. Proceedings of the National Academy of Sciences of the United States of America, 105(3), 1044–1049. doi:10.1073/pnas.0706446105
(61) Cribbet et al (2014). Cellular aging and restorative processes: subjective sleep quality and duration moderate the association between age and telomere length in a sample of middle-aged and older adults. Sleep, 37(1), 65–70. doi:10.5665/sleep.3308
(62) Walker, M. (2018). Why we sleep : The new science of sleep and dreams. London, UK: Penguin Books.
(63) Santhi, N et al (2012). The spectral composition of evening light and individual differences in the suppression of melatonin and delay of sleep in humans. Journal of Pineal Research, 53(1), 47-59. doi:10.1111/j.1600-079X.2011.00970.x
(64) Green et al (2017). Evening light exposure to computer screens disrupts human sleep, biological rhythms, and attention abilities. Chronobiology International, 34(7), 855-865. doi:10.1080/07420528.2017.1324878
(65) Gottesmann, C. (2002). Gaba mechanisms and sleep. Neuroscience, 111(2), 231-239. doi:10.1016/S0306-4522(02)00034-9
(66) Dinges et al (1997) Cumulative sleepiness, mood disturbance, and psychomotor vigilance performance decrements during a week of sleep restricted to 4-5 hours per night. Sleep, 20(4):267-277
