Since the discovery of rapid eye movement (REM) sleep in 1953, the scientific study of sleep and its importance has grown rapidly in tandem with an acceptance of the prevalence and impact of insufficient sleep.
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Progress in sleep science has been driven by advances in our understanding of both circadian biology and the biological basis of sleep, allowing a consensus among researchers regarding the amount of sleep necessary for good health.
Although this varies slightly between individuals, most healthy adults require between seven and nine hours per day. Sleep requirements for children and teenagers are higher: teenagers need eight to ten hours, children need between ten and fourteen hours per day, whilst infants require up to seventeen hours of sleep per 24-hour period.
Quantity and quality of sleep affects a multitude of physical and mental domains. It plays vital roles in energy conservation (recovering from the fatigue of being awake and active), brain development, immune responses, cognition and functioning and clearance of ‘brain waste’, the by-products of cellular respiration in the neurons of the brain.
What are the major functions of sleep?
Learning and consolidation
During our waking hours, we are constantly ‘learning’ from information in our environment which allows us to adapt to our surroundings and ultimately survive. The ability of our brain to learn in this manner is referred to as brain plasticity and occurs when there is a strengthening of the communicative links (synapses) between neurons.
However, as the synapses strengthen, they require more energy and begin to react less selectively to stimuli leading to a saturation of the ability to learn.
During sleep, the brain is no longer connected to the environment: essentially the brain goes ‘offline’ and can go through the process of synaptic renormalization, whereby the strengthened synapses return to a synaptic homeostasis. This prevents an exponential growth in synapse strength, which if left unchecked would lead to saturation, impairing learning.
Evidence for this theory of why sleep is so critical to learning has come from studies of the brains of mice. It is established that stronger synapses are larger, therefore if the brain does pass through a process of synapse strengthening during wakefulness and synapse normalization during sleep, there should be a measurable difference in synapse size in sleep compared to waking states.
Researchers have found that the synapses of mice were 18% smaller following a six to eight-hour period of sleep compared to a similar period awake.
One recent study in humans examined the effect of unbroken versus broken sleep on the ability to learn new tasks. Participants were asked to perform a series of movement learning tasks after a night of unperturbed sleep versus a night where researchers interrupted their deep sleep. After the interrupted sleep, participants made many more errors. Crucially, the participants were unaware of a difference in sleep quality between the nights.
Physical health
Poor sleep can have a profound impact on long-term physical health. Cross-sectional studies that examine the relationships between sleep habits and the existence of physical illnesses have shown associations between sleep loss and obesity, heart disease, diabetes and immune functioning.
As numerous studies have linked body mass index (BMI) and sleep, insufficient sleep is now recognized as a potential risk factor for obesity, with those regularly sleeping for less than six hours a day having an above-average BMI.
Poor sleep could lead to behaviors that increase overall caloric intake, for example choosing high calorie-density foods for a quick energy boost or a lack of energy to participate in calorie-burning exercise.
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Research has also identified changes in the secretion of hormones that affect weight gain following insufficient sleep. Poor sleep has been linked to the underproduction of leptin, a hormone that suppresses appetite and the over-production of ghrelin, an appetite-stimulating hormone. In addition, too little sleep stimulates the promotion of cortisol which is shown to stimulate appetite and preferences for high fat and calorie-dense foods.
One serious health consequence of insufficient sleep is an elevated risk of diabetes. Poor sleep appears to contribute to insulin resistance, where the body becomes less sensitive to insulin and begins to secrete the hormone at higher levels to control blood sugar. Eventually, this could result in Type 2 diabetes which is usually associated with consuming a high-fat diet. However, one recent study demonstrated that one night of complete sleep deprivation can have the same impact on insulin resistance as six months of consuming a high-fat diet.
Quality and quantity of sleep also affect immune system functioning. During periods of sleep, the body releases cytokines, a type of protein secreted by cells of the immune system. During bodily infection or systemic inflammation, cytokines increase to promote recovery. Sleep deprivation may reduce the production of these cells.
Mental health
The relationship between sleep and mental health is complex. Some psychiatric disorders such as schizophrenia can cause severe sleep disturbance, and poor sleep can exacerbate the symptoms of pre-existing psychiatric conditions. Overall, disturbances of sleep correlate in some way with most mental disorders. More recently, prospective research has indicated the direction of causality between sleep and mental distress. One meta-analysis of 21 studies demonstrated that people with insomnia have a two-fold risk of subsequently developing depression compared to those without.
Additionally, one large-scale study looking at the clinical effectiveness of cognitive-behavioral therapy (CBT) found that those who were given CBT specifically for insomnia showed significantly greater improvements in depressive, paranoid and anxious symptoms than those who did not receive insomnia-specific CBT.
References
Baglioni, C., Battagliese, G., Feige, B., Spiegelhalder, K., Nissen, C., Voderholzer, U., Lombardo, C. and Riemann, D., 2011. Insomnia as a predictor of depression: A meta-analytic evaluation of longitudinal epidemiological studies. Journal of Affective Disorders, 135(1-3), pp.10-19.
Chao, A., Jastreboff, A., White, M., Grilo, C. and Sinha, R., 2017. Stress, cortisol, and other appetite-related hormones: Prospective prediction of 6-month changes in food cravings and weight. Obesity, 25(4), pp.713-720.
Cirelli, C. and Tononi, G., 2008. Is Sleep Essential?. PLoS Biology, 6(8), p.e216.
Medicalnewstoday.com. 2020. How Does Poor Sleep Affect Our Ability To Learn? Study Investigates. [online] Available at: <https://www.medicalnewstoday.com/articles/317597#Poor-sleep-keeps-synapses-excited,-blocks-the-brains-ability-to-learn> [Accessed 10 November 2020].
ScienceDaily. 2020. Insulin Sensitivity: One Night Of Poor Sleep Could Equal Six Months On A High-Fat Diet, Study In Dogs Suggests. [online] Available at: <https://www.sciencedaily.com/releases/2015/11/151104134039.htm#:~:text=Broussard.,diseases%20like%20obesity%20and%20diabetes.%22> [Accessed 10 November 2020].
Shepard JW Jr, Buysse DJ, Chesson AL Jr, et al. History of the development of sleep medicine in the United States. J Clin Sleep Med. 2005;1(1):61-82.
Further Reading
Is healthy sleep, both before and during the COVID-19 pandemic, prior to SARS-CoV-2 infection, protective against post–COVID-19 condition?