Sleep is at once highly familiar — we all do it — and deeply mysterious; among those studying the science of sleep, there is no consensus on why it evolved in the first place (Assefa et al., 2015).
Fortunately, in recent decades, the science of sleep has moved forward. New technologies have allowed scientists to study the changes that take place in the brain and body during sleep, revealing more of what happens and why.
Read on to learn more about these research findings and how to use it to improve sleep.
Before you continue, we thought you might like to download our three Positive Psychology Exercises for free. These science-based exercises explore fundamental aspects of positive psychology, including strengths, values, and self-compassion, and give you the tools to enhance the wellbeing of your clients, students, or employees.
To understand the science of sleep, let’s first grasp the basic processes that regulate our sleep.
Biological processes
Research into the science of sleep has identified two biological processes that govern sleep: circadian rhythm and sleep–wake homeostasis (Dijk & Czeisler, 1995).
Circadian rhythms are the daily cycles in a range of bodily functions (e.g., temperature, metabolism, and the release of hormones) that cause us to wake up in the morning and to feel sleepy in the evening (Albrecht, 2012).
Circadian rhythm is maintained by a kind of biological clock in the body and brain and is set to a precise 24-hour cycle each day by environmental cues such as changes in light levels (Czeisler et al., 1999).
Sleep–wake homeostasis, meanwhile, tracks how long it has been since you last slept and regulates both the desire to sleep and the intensity of sleep (Borbély, 1982). The longer you have been awake, the greater your “sleep drive” and the more deeply you will sleep.
What Are the 5 Sleep Stages?
There are five stages of sleep that the body moves through repeatedly during the night. Research into the science of sleep measures these stages by tracking brain activity and eye movements (Patel et al., 2022).
Each time we pass through all five of the stages, we have completed a “sleep cycle,” which takes an average of 90 minutes (Memar & Faradji, 2017). The five sleep stages are N1, N2, N3, N4, and REM (Patel et al., 2022).
The four N stages are non-REM or dreamless sleep, with each one being a deeper form of sleep than the last. REM stands for “rapid eye movement,” where (as you might imagine) the eyes move rapidly; this is associated with dreaming (Patel et al., 2022).
As the night moves on through a series of sleep cycles, the body spends progressively more time in REM sleep and less in non-REM sleep, especially the deepest N3 stage (Patel et al., 2022).
Here’s a video summary of the five sleep stages:
A walk through the stages of sleep - TED
What happens during REM sleep?
Brain activity ramps up in REM sleep and is roughly the same, when measured by EEG, as during wakefulness. This correlates with increased use of oxygen by the brain relative to the other sleep stages. Pulse and blood pressure also increase, but their levels vary throughout REM sleep (Patel et al., 2022).
The muscles, meanwhile, are paralyzed, apart from those that generate the rapid eye movements that give this sleep stage its name (Patel et al., 2022). These movements are thought to be related to dreaming (Dement & Kleitman, 1957).
How Sleep Affects Your Physical & Mental Wellbeing
While we may not have the answer to the fundamental question, “Why do we sleep?” research into the science of sleep has revealed several specific benefits that it brings us. Let’s take a look at them.
Restoration
Sleep is important for physical growth and repair; muscle growth, protein synthesis, tissue repair, and the release of growth hormones happen mostly or only while we are asleep (Oswald, 1980).
In a similar vein, sleep can rejuvenate and restore the brain. Most obviously, sleep stops us from feeling tired because during sleep, our brains are cleared of adenosine, a by-product of the brain’s activity that contributes to feelings of tiredness (Huang et al., 2011).
Chemicals that are toxic to the brain might also be cleared during sleep via a drainage system in the brain known as the glymphatic system (Chong et al., 2022).
Metabolism
We undergo metabolic changes during sleep, including a reduction in the rate of metabolism and body temperature during non-dreaming sleep. (Copinschi et al., 2014). These changes help us to maintain a healthy weight and avoid diseases such as diabetes and heart disease (Copinschi et al., 2014).
Mood and mental health
The impact of sleep on our state of mind is obvious: Most of us feel a lot better when we are well rested. But as research into the science of sleep reveals, this effect goes beyond ordinary fluctuations in mood. Every category of mental illness is associated with sleep disturbance, and treatment for sleep problems results in improved mental health (Anderson & Bradley, 2013).
Cognitive functions
Again, most of us feel sharper after a good night’s sleep, and research into the science of sleep has confirmed that our performance on learning and cognitive tasks is affected by a lack of sleep (see below). This has led researchers to suggest that one of the main functions of sleep might be neural plasticity — changes to the structure and organization of the brain during sleep (Wang et al., 2011).
Research Findings on Sleep Deprivation
Much of what we know about the science of sleep has come from research into what happens when we don’t get enough sleep. Let’s see what that research says.
Obesity
Sleeping too little has been repeatedly linked to weight gain (Patel & Hu, 2008). This is likely because sleep deprivation upsets the balance of hormones that we secrete during sleep that help control our appetite and metabolism. As a result, we both eat more calories when we don’t get enough sleep and store more of them as fat (Spiegal, 2004; Ding et al., 2018).
Diabetes
Sleep deprivation may increase the risk of type 2 diabetes by slowing the rate at which the body processes glucose, as shown in several prospective studies about the science of sleep (Larcher et al., 2015).
Heart disease and hypertension
There is a correlation between sleep deprivation and cardiovascular disease and stroke, and this might be due to increased blood pressure (Nagai et al., 2010). Studies have found that sleeping for less than five hours per night increases the risk of hypertension (Gangwisch et al., 2006), and sleeping for seven or fewer hours per night (but also for nine or more) increases the rate of coronary heart disease (Ayas et al., 2003).
Mood
There is a clear link between sleep deprivation and problems with mood. In one study, participants who were restricted to 4.5 hours of sleep per night reported feeling more stressed, sad, angry, and mentally exhausted (Dinges et al., 1997).
In another large-scale study, those who experienced chronic insomnia were found to be five times more likely to develop depression (Neckelmann et al., 2007).
Immunity
Most of us will have noticed that illness makes us feel tired and sleep more, which leads to the obvious inference that sleep may be necessary for the proper functioning of the immune system (Besedovsky et al., 2019).
According to Besedovsky et al. (2019), evidence from both humans and animals supports this inference in the field of sleep science.
Mental performance
Researchers in the science of sleep have found that mental performance declines along a number of dimensions when we are sleep deprived. Alertness, concentration, working memory, mathematical capacity, and logical reasoning are all affected (Alhola & Polo-Kantola, 2007).
Life expectancy
As might be expected given the above, poor sleep is associated with a reduced life expectancy. Three large cross-sectional epidemiological studies have found that sleeping for five hours or less per night is associated with a 15% increase in risk of mortality (Altevogt & Colten, 2006).
While the science of sleep has a lot to offer in terms of promoting wellbeing, there are some caveats, in terms of the limitations of research into the science of sleep and the myths that have grown up around it.
Problems with the research
First, sleep is not an easy thing to study, and so what we know about the science of sleep is not as robust as we might wish.
Until the recent advent of wearable fitness tracking devices, it has been very hard to directly measure the sleep of large numbers of people; they would all have had to sleep in a laboratory setting. This means that much of what we know about the science of sleep depends on participants reporting on their own sleep. Self-reported data, however, can be unreliable (Althubaiti, 2016).
Apart from experiments using self-reported data, much of the rest of the research that makes up the science of sleep is cross-sectional, meaning that correlations are sought between sleep and various health outcomes. But correlation does not equal causation, so it is not clear whether poor sleep is correlated with a particular outcome, or if it is the cause of that outcome.
The sleep loss epidemic
There is a popular belief that people in industrialized countries sleep less, on average, than we would have in our pre-industrial past, and that this is causing health problems (Bin et al., 2013).
Research into the science of sleep, though, suggests that this might be false. Research carried out with hunter–gatherer communities in Namibia, Tanzania, and Bolivia living as our ancestors would have can shed light on how much sleep is enough. This research found that those hunter–gatherers slept for an average of 6.5 hours per night, which is, if anything, a lower average than that found in industrialized countries (Siegel, 2022).
3 Common Sleep Disorders
Some of us don’t get enough sleep, but not always for the same reasons. Here, the science of sleep overlaps with psychiatry and psychology.
Let’s look at three of the most common sleep disorders.
Insomnia
This is the most common sleep disorder, and it consists of an inability to sleep, which can take the form of either difficulty falling asleep or waking at night (World Health Organization, 2019).
Quite often, chronic insomnia is caused by the person’s reaction to an initial bout of insomnia. Once insomnia begins, people might become agitated and anxious about the process of falling asleep, which then makes it harder to fall asleep. A vicious cycle of insomnia and agitation is established (Lundh et al., 1991).
Insomnia can be treated in the short term with sedative medication and in the short and long term with cognitive behavioral therapy (Koffel et al., 2018; Matheson & Hainer, 2017).
Restless legs syndrome
In this condition, individuals experience unpleasant sensations in the legs, such as pain or a “creeping” feeling, which may be relieved by moving the leg (World Health Organization, 2019). This gets in the way of falling asleep.
There is evidence from research about the science of sleep that restless legs syndrome might be linked to abnormalities in the neurotransmitter dopamine, and it is often treated with medications that address this abnormality (Allen, 2004).
Sleep apnea
Sleep apnea is a difficulty in breathing while asleep that causes breathing to stop temporarily before restarting with a gasping or snoring sound (World Health Organization, 2019).
This results in poor sleep, together with potentially serious complications such as hypertension, heart disease, cognitive deficits, and accidents (Harding, 2000).
Treatments include weight loss, changing the sleeping position, over-the-counter medications, specialized devices to increase air pressure in the individual’s airway, and surgery (Chang et al., 2020).
5 Signs You Are Not Getting Enough Sleep
The most obvious sign of insufficient sleep is … feeling tired! But for those of us who need a few additional pointers (drawn more from common sense than the science of sleep), you could also look out for the following:
Needing an alarm clock to wake up
Sleeping in on weekends
Falling asleep during the day
Struggling with memory and problem-solving
Looking tired, with bags under your eyes
How to Use the Science of Sleep to Improve Your Sleep
There are a number of simple but evidence-based strategies that we can use to improve our sleep, which together are sometimes called “sleep hygiene.”
Avoid alcohol. Many people believe that alcohol helps them sleep, but research into the science of sleep reveals that it actually reduces sleep quality.
Avoid screens and artificial lights in the evening (Tsouklidis et al., 2020). Our bodies’ natural rhythms are set by the rising and setting of the sun, so beaming artificial light into our eyes after dark can trick our brains into thinking that it’s not time to sleep yet.
Don’t drink caffeine in the evening. Caffeine suppresses the action of adenosine, a chemical that makes us feel tired.
Sleep in a quiet room.
Don’t catch up on sleep. If you experience fragmented sleep, you might be tempted to sleep in longer in the mornings or to nap, but this just solidifies your pattern of broken sleep. Instead, stick to a set amount of time for sleep each night.
How positive psychology can help you sleep better
As with other areas of medicine and psychology, the science of sleep often focuses on things that go wrong rather than right. And as in other areas, positive psychology has an alternative vision to offer: a picture of how positive mental states can contribute to good sleep and quality of life.
Gratitude
A 2020 systematic review found that in five out of eight studies included, interventions to promote gratitude also led to subjectively better sleep (Boggiss et al., 2020).
Self-compassion
A 2021 meta-analysis of 17 studies found an association between self-compassion and self-reported sleep quality (Brown et al., 2021).
Optimism
A 2019 study found that there was a significant association between optimism and self-reported sleep quality (Hernandez et al., 2020).
17 Top-Rated Positive Psychology Exercises for Practitioners
Expand your arsenal and impact with these 17 Positive Psychology Exercises [PDF], scientifically designed to promote human flourishing, meaning, and wellbeing.
A key part of a good night’s sleep is a good bed. Use our bed checklist to check whether yours is up to the job.
Sleep hygiene is one of the ways, according to the science of sleep, that you can enhance your sleep. Use this sleep hygiene checklist to see where yours could be improved.
The first step in improving your sleep is to get a clear picture of how it is at the moment. You can do this with our sleep diary.
If you wake up often in the night or for a long time, then you need to restrict your sleep — a cognitive-behavioral technique for consolidating your sleep. The sleep restriction guide walks you through it.
While sleep remains mysterious — no one is quite sure why we do it in the first place — what is clear is that it is hugely important. Good-quality sleep should be a priority for us all.
Research about the science of sleep is revealing ever more about our sleep needs and how to meet them, including, perhaps, the good news that more of us are getting adequate sleep.
With some sensible sleep hygiene, it seems that most of us don’t need to let anxiety about sleep keep us up at night.
Women tend to sleep more than men due to differences in brain structure and hormonal cycles, which may increase their need for recovery through sleep. Hormonal changes during menstruation, pregnancy, and menopause can also disrupt sleep patterns, making women more prone to longer sleep durations and increased fatigue. Women are more likely to experience insomnia and sleep disturbances, further influencing the amount of rest they require.
What is the psychology behind sleep?
Sleep has profound psychological effects, impacting mood regulation, memory consolidation, and emotional processing. A lack of sleep is associated with higher rates of anxiety, depression, and stress, as well as cognitive impairments like difficulty focusing and reduced problem-solving skills. Sleep also plays a critical role in maintaining emotional balance, making it an essential aspect of mental health and psychological wellbeing.
How many hours of sleep should you aim for?
For optimal health, adults are recommended to get between 7 to 9 hours of sleep per night. This range ensures that the body and brain have enough time to perform critical functions such as tissue repair, hormone regulation, and memory processing. Chronic sleep deprivation can lead to long-term health problems, including cardiovascular diseases, obesity, and weakened immune responses.
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About the author
Dr. Michael Eisen, Ph.D., is a Clinical Psychologist, mindfulness trainer, advisor to mental health startups, and Associate Clinical Tutor at UCL. As a therapist in private practice, Michael draws on a range of methods, especially mindfulness-based therapies.
Sleep is at once highly familiar — we all do it — and deeply mysterious; among those studying the science of sleep, there is no consensus on why it evolved in the first place (Assefa et al., 2015).
There are five stages of sleep that the body moves through repeatedly during the night. Research into the science of sleep measures these stages by tracking brain activity and eye movements (Patel et al., 2022).
Much of what we know about the science of sleep has come from research into what happens when we don’t get enough sleep. Let’s see what that research says.
Some of us don’t get enough sleep, but not always for the same reasons. Here, the science of sleep overlaps with psychiatry and psychology.
There are a number of simple but evidence-based strategies that we can use to improve our sleep, which together are sometimes called “sleep hygiene.”
