What is REM sleep?
Sometimes during sleep, the brain will also revert to rapid eye movement, or REM sleep. Discovered by Kleitman and his student Eugene Aserinsky, REM sleep is characterized by rapid eye movement behind the lids, and brain patterns during REM sleep closely resemble those of the awake brain. It is also the stage of sleep where dreams occur.
The brain will go through these various stages of sleep several times a night, taking about 90 minutes to two hours to complete a single cycle. All these cycles are important for human health.
What conditions have been linked to sleep?
One of the most popular theories as to why sleep is so important is that it’s essential in 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 illness.
There’s also a growing body of evidence that sleep helps the brain clear out cellular and protein debris that could otherwise build up and cause damage, leading to inflammation and cell death.
Over 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.
Since the mid-20th century, researchers have begun to uncover many critical links between sleep and metabolic health, particularly strong relationships between sleep duration and quality and weight, heart health, and metabolic diseases such as diabetes. Part of this work was led by University of Chicago professor Eve Van Cauter, who was among the first to discover that sleep loss is bad for the body and the brain.
A 2015 study by Esra Tasali, director of the UChicago Sleep Center and associate professor of medicine at UChicago Medicine, linked poor sleep quality and shortened sleep duration to increased risk of diabetes and obesity.
His work uncovered some of the mechanistic pathways underlying these relationships, finding that sleep restriction results in the inability of insulin to regulate blood sugars in part due to impaired fat metabolism, as well as demonstrating that the inadequate sleep leads to higher levels of hormones that regulate appetite and increased calorie intake.
In related research, Erin C. Hanlon, assistant professor of medicine, and colleagues studied the relationship between sleep loss and food choices, finding that sleep-deprived study participants had higher levels of chemical cues than may make high-calorie, high-fat foods more enjoyable than when they weren’t sleep deprived, and that after sleep deprivation, the subjects ate nearly twice as much carbohydrates and fat.
With the close relationship between sleep, hunger cues, and glucose processing, it’s perhaps not surprising that inadequate and disrupted sleep has also been linked to a higher risk of diabetes and heart problems. Research from the Tasali lab and others has found that when sleep is deprived, the body has a harder time regulating blood sugar levels due to elevated levels of fatty acids, which interfere with insulin processing.
Poor sleep quality has also been linked to higher resting heart rate, high blood pressure, heart attack and stroke.
A common adage when someone is sick is to tell them to “get plenty of rest,” so perhaps it’s no surprise that there are strong links between sleep duration, sleep quality, and immune function. Lack of adequate sleep has been linked to an increased risk of infection and inflammation, and research has shown that not enough sleep increases the chances of catching common illnesses like a cold or flu.
Sleep is important for recovering from illness and injury, and research has found that increased levels of inflammatory signaling promote sleep. Sufficient, high-quality sleep is linked to stronger innate and adaptive 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, more protective immune response against dangerous antigens in the future.
The body actually increases inflammation during the night, which is thought to be important for boosting adaptive immunity. During sleep, the body has mechanisms in place to protect itself from this inflammation, such as the production of the hormone melatonin. In situations where people don’t get enough sleep, the inflammation may persist after waking up. Long-term systemic inflammation is associated with a variety of health problems, such as diabetes, cardiovascular disease, and even depression and cancer.
A growing body of research has linked a lack of quality sleep with poor cognitive function, cognitive decline and dementia in later life, further reinforcing the need for adequate sleep throughout life.
During sleep, and especially slow wave sleep, the body’s metabolic activities slow down, allowing it to engage in reparative processes. Research in mice has found that slow-wave sleep leads to increased flow of cerebrospinal fluid throughout the brain, allowing it to clear itself of debris while we rest. This can be especially important for the elimination 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 linked to sleep, particularly our declarative memory, the type of memory that allows us to recall specific facts and events. Slow-wave sleep is thought to be critical in enabling the hippocampus, which processes short-term memory, to convert information into long-term memories in the neocortex.
Diane Lauderdale, the Louis Block Professor and chair of the Department of Public Health Sciences at the University of Chicago, has conducted a series of studies contrasting older adults’ subjective experience of sleep quality with more objective measures of sleep. A 2019 study linked more objectively measured sleep disruption from 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 as measured objectively by actigraphs, but that only subjective loneliness is associated with self-reported symptoms of insomnia.
Can you improve your health by getting more sleep?
Research has shown repeatedly that improving sleep quality and achieving a healthy sleep duration can lead to improvements on all kinds of other health measures and outcomes. It is worth identifying and addressing the underlying causes of poor quality sleep and short sleep duration.
While poor sleep has been linked to heart problems and other metabolic issues, 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 may improve sleep quality and reduce the risk of metabolic and cardiovascular disease in individuals, and may prevent patients with prediabetes from progressing to diabetes clinical. CPAP treatment can also lower the resting heart rate in prediabetes patients with sleep apnea, a measure related to better heart health. Just catching up on sleep lost on the weekends can reduce your overall risk of diabetes.
Tasali and his team at the UChicago Sleep Center are focused on better understanding how improving sleep quality and duration can improve cardiovascular and metabolic health and overall health outcomes, and some of the solutions may be surprisingly simple. . In a 2022 study, his team, along with colleagues at the University of Wisconsin-Madison, found that providing personalized sleep hygiene counseling led study participants to sleep an extra hour a night, and that additional sleep was associated with a reduction in daily calorie intake.