Learn more about breakthroughs pioneered at the University of Chicago

How sleep affects human health, explained

Editor’s note: This is part of a series called “The Day Tomorrow Began,” which explores the history of breakthroughs at UChicago. Learn more here.

All human beings need sleep. We spend a third of our lives asleep, and it’s as important as food, water or air to our survival. While scientists are still working to understand the exact nature of sleep and why it’s so important to our well-being, decades of research have made it clear that adequate sleep is vital to our physical and mental health. Among many other things, sleep plays a crucial role in memory consolidation and other brain functions, supporting the immune system and healing after injury or disease, and protecting against heart disease and diabetes.

How does sleep affect human health?

All human beings — and almost all animals — need sleep. We spend a third of our lives asleep, and it’s as important as food, water or air to our survival.

Typically, sleep is defined as an altered state of consciousness or a condition of the body and mind wherein the nervous system is less active, the body is relaxed, and consciousness is suspended. The definition of sleep may seem obvious — everyone knows the difference between being asleep and being awake. But scientifically and medically speaking, the term is less clear, and there is still a lot we don’t understand about the phenomenon.

While scientists are still working to understand the exact nature of sleep and why it’s so important to our well-being, decades of research have made it clear that adequate sleep is vital to our physical and mental health.

Researchers around the world — including those at the University of Chicago, home to the world’s first sleep research lab, opened in 1925 — have helped to clarify major questions about the biological functions of sleep, as well as its importance in consolidating memories and protecting the immune system, and the negative impacts of sleep deprivation.

Why do we sleep?

Sleep is literally critical to our survival. We know this thanks to the groundbreaking work of UChicago Prof. Allan Rechtschaffen, a sleep research pioneer and longtime director of the UChicago Sleep Laboratory.

Rechtschaffen was driven by a desire to understand why we sleep. All mammals, birds and reptiles sleep in some form or another, and nearly all animals exhibit some sort of resting or sleep-like behavior. But how and why is it beneficial to any species to spend a significant portion of its life unconscious, unable to search for food or shelter, to reproduce or to avoid predators?

In 1983, Rectschaffen made a major discovery: without sleep, a mammal will die. Rats that were continuously deprived of sleep began to suffer severe health effects and died within about two weeks. His work led to deeper exploration into understanding why sleep is so critical to survival, though despite decades of research, the answer is still unclear.

What happens to the brain during sleep?

In the brain, sleep manifests as changes in brain activity. Much of what we understand about the stages of sleep was learned thanks to the early sleep research done by UChicago Prof. Nathaniel Kleitman, founder of the world’s first laboratory focused exclusively on sleep. His 1939 textbook, Sleep and Wakefulness, has long been considered a foundational text for sleep researchers around the world. His work using all-night electroencephalogram (EEG) recordings allowed Kleitman and his team to chart the sequence of sleep patterns in the brain over the course of the night.

The neuronal cells of the brain use electrical signaling to send and receive messages throughout the brain and body. During the transition into sleep, those electrical signals begin to fall into wave-like patterns, with different wave patterns associated with different stages of sleep. As the body relaxes, the brain shifts into the relatively rapid alpha waves. As a person dozes off, the signaling transitions into slower theta waves, before finally settling into the deepest stage of slow wave sleep.

What is REM sleep?

At times during sleep, the brain will also shift back up into rapid eye movement, or REM, sleep. Discovered by Kleitman and his student Eugene Aserinsky, REM sleep is characterized by rapid movement of the eyes behind the eyelids, and brain patterns during REM sleep most closely resemble those of the awake brain. It’s also the stage of sleep where dreams occur.

The brain will cycle through these various stages of sleep several times per night, taking about 90 minutes to two hours to complete a single cycle. All of these cycles are important for human health.

What conditions have been linked to sleep?

One of the most popular theories about why sleep is so important is that it’s critical for allowing the body to grow, repair and rejuvenate itself. Sleep is vital for memory consolidation as well as supporting normal immune function and healing after injury or disease.

There is also a growing body of evidence that sleep helps the brain to clear cellular and protein debris that can otherwise build up and cause damage, leading to inflammation and cell death.

In the past century, scientists have recognized several sleep disorders, such as narcolepsy, as well as the deleterious effects of sleep deprivation and poor-quality sleep on our long-term health.

These include:

    Since the mid-20th century, researchers have begun to uncover many critical links between sleep and metabolic health — in particular, there are strong relationships between sleep duration and quality and weight, heart health, and metabolic diseases such as diabetes. Some of this work was spearheaded by UChicago Prof. Eve Van Cauter, who was among the first to discover that sleep loss is bad for the body as well as the brain.

    A 2015 study from Esra Tasali, Director of the UChicago Sleep Center and an Associate Professor of Medicine at UChicago Medicine, has connected poor sleep quality and reduced sleep duration to increased risk of diabetes and obesity.

    Her work has uncovered some of the mechanistic pathways behind these relationships, finding that sleep restriction results in inability of insulin to regulate blood sugars partly due to impaired fat metabolism, as well as showing that inadequate sleep leads to higher levels of hormones that regulate appetite and increased caloric intake.

    In related research, Erin C. Hanlon, Assistant Professor of Medicine, and colleagues have studied the relationship between sleep loss and food choices, finding that sleep-deprived study participants had higher levels of chemical signals that can make high-fat, high-calorie foods more pleasurable than when not sleep-deprived and that following sleep deprivation subjects ate nearly twice as much carbohydrates and fat.

    With the tight relationship between sleep, hunger cues, and glucose processing, it’s perhaps not surprising that inadequate and disrupted sleep has also been connected to a higher risk of diabetes and heart problems. Research from the Tasali lab and others has found that when sleep deprived, the body has a harder time regulating blood sugar levels due to elevated fatty acid levels, which interfere with insulin processing.

    Poor sleep quality has also been linked to a higher resting heart rate, high blood pressure, heart attack and stroke.

    A common adage when someone is ill is to tell them to get “plenty of rest,” so it is perhaps not surprising that there are strong ties between sleep duration, sleep quality and immune function. A lack of adequate sleep has been linked to increased risk of infection and inflammation, and research has shown that insufficient sleep makes it more likely that you’ll catch common illnesses such as a cold or the flu.

    Sleep is important for recovering from illness and injury, and research has found that increased levels of inflammatory signaling promotes sleep. Sufficient high-quality sleep is connected to stronger adaptive and innate immunity, reduced allergic reactions, and a more efficient response to vaccines.

    Similar to how sleep supports memory consolidation, sleep is thought to strengthen immune memory, allowing the body to generate a stronger and more protective immune response against dangerous antigens in the future.

    The body actually ramps up inflammation at night, which is thought to be important for strengthening adaptive immunity. While sleeping, the body has mechanisms to protect against this inflammation, such as the production of the hormone melatonin. In situations where people aren’t getting enough sleep, that inflammation can linger after waking up. Long-term, systematic inflammation is associated with a host of health issues, such as diabetes, cardiovascular disease, and even depression and cancer.

    A growing body of research has connected a lack of quality sleep with poor cognitive function, cognitive decline and dementia in old age, further bolstering the need for adequate sleep throughout the lifetime.

    During sleep, and particularly slow-wave sleep, the body’s metabolic activities slow, allowing it to engage in restorative processes. Research in mice has found that slow-wave sleep leads to an increased flow of cerebrospinal fluid throughout the brain, allowing it to cleanse itself of debris while we rest. This may be especially important for the clearance of waste products associated with dementia. For example, disrupted sleep leads to increased levels of amyloid-β, a protein strongly linked to Alzheimer’s disease.

    Memory processing and consolidation are also closely tied to sleep, particularly our declarative memory — the type of memory that allows us to remember specific facts and events. It is thought that slow-wave sleep is critical for allowing the hippocampus, which processes short-term memory, to convert information into long-term memories in the neocortex.

    Diane Lauderdale, Louis Block Professor and Chair of the Department of Public Health Sciences at UChicago, has carried out a number of studies that contrast older adults’ subjective experience of their sleep quality with more objective measures of sleep. A 2019 study connected greater sleep disruption as objectively measured by a wearable actigraph device with worse cognitive function and greater cognitive decline over five years in older adults. Interestingly, participants’ self-reported insomnia symptoms, a subjective measure of more disrupted sleep, did not show the same correlation with cognition.

Other research among older adults has shown that both feeling lonely and being socially isolated are associated with more disrupted sleep measured objectively by actigraphs, but that only subjective loneliness associated with self-reported insomnia symptoms.

Can you improve your health by sleeping more?

Research has shown again and again that improving sleep quality and attaining a healthy sleep duration can lead to improvements on all kinds of other health measures and outcomes. It’s worth it to identify and address the underlying causes of poor-quality sleep and low sleep duration.

While poor sleep has been connected to heart issues and other metabolic problems, improving sleep habits has been shown to improve measures of many of these conditions. For example, receiving continuous positive airway pressure (CPAP) treatment for sleep apnea can improve sleep quality and lower the risk for metabolic and cardiovascular diseases in individuals and can prevent patients with prediabetes from progressing to clinical diabetes. CPAP treatment can also lower resting heart rate in prediabetes patients with sleep apnea, a measure correlated with better heart health. Even just catching up on lost sleep on the weekends can reduce overall diabetes risk.

Tasali and her team at the UChicago Sleep Center are focused on better understanding how improving sleep quality and duration can improve metabolic and cardiovascular health and overall health outcomes, and some of the solutions may be surprisingly simple. In a 2022 study, her team, along with colleagues at the University of Wisconsin-Madison, found that providing personalized sleep hygiene counseling led to study participants sleeping an hour more per night — and that that additional sleep was associated with a reduction in daily caloric intake.

Perhaps obviously, sleep deprivation and poor sleep quality can lead to increased risk of injury and falls, particularly among older adults and those who are ill; this is one reason that researchers such as Vineet Arora, the Herbert T. Abelson Professor of Medicine and dean for Medical Education at UChicago, have led efforts to understand how sleep disruptions affect the health and wellbeing of hospitalized patients, as well as developing new strategies for reducing the number of nighttime awakenings and sleep interruptions for those patients.

What is good sleep hygiene?

In the modern era, surrounded by light-up devices and all kinds of responsibilities and entertainment that demand our attention, it can be challenging to maintain good sleep hygiene, which can contribute to loss of sleep and poor sleep quality.

Good sleep hygiene includes strategies such as developing a bed time routine that helps you wind down for the evening, limiting the use of electronic devices near bed time, keeping the bedroom dark and cool, and going to bed and waking up at the same time each day. These strategies can improve overall sleep quality and, in turn, contribute to better overall health.

Sleep is still poorly understood, even after decades of research; it’s not quite clear what exactly sleep is, or why our brains and bodies need it, but it is clear that it plays a vital role in our health. Continued research into its role in our lives, as well as how to optimize our sleep and sleep conditions, will help pave the way for better health for many.