Doctors are starting to realize the importance of getting adequate sleep. The American Heart Association advised physicians in 2022 to consider asking patients about their sleep duration as a critical factor, as significant as checking their smoking status, blood pressure, diet, and exercise habits.

But many of us routinely force our bodies to fight sleep. We drink caffeinated beverages by day to stay alert. Some of us work all night and sleep in daylight. Others switch time zones and struggle with jet lag.

At the same time, neuroscientists are realizing that the timing of our sleep may be just as important as the amount. We are beginning to understand how we might work with, rather than against, the rhythms of our bodies. For example, new research suggests these rhythms may play a role in patients’ responses to cancer treatment.

Body clocks

For millions of years, humans have experienced a consistent pattern of light exposure, with minimal day-to-day variability in the timing of sunset and sunrise. So our genes prepared us to be active in daylight and rest after nightfall.

In 1959, physician Franz Halberg was the first to document our roughly 24-hour self-regulating systems – that he called circadian rhythms – from the Latin words for "about" and "day." In 2017, three scientists won a Nobel Prize for showing that humans have a network of timekeeping genes and proteins with daily cycles of rest and activity.

These "molecular clocks" in our brain and in every cell modulate our body systems, including hormones, blood pressure, blood glucose, and body temperature.

One circadian loop: We are genetically programmed to be hungrier in the evening, which helps us load up on calories before the overnight fast, and less hungry early in the morning, so we can use our energy reserves to go out and hunt for our next meal.

The loop for sleep balances the "sleep drive" with the need to be awake. When we've been awake for a long time, our sleep drive kicks in and tells us we need to sleep.

People whose lives are synchronized with their body clock signals are less fatigued, have better moods, maintain healthier weights, gain more benefit from their medications, think more clearly, and have improved long-term health outcomes, said neuroscientist Russell Foster, who heads the Sleep and Circadian Neuroscience Institute at Oxford University in the U.K.

Naturally early risers, or morning "larks," do best when they can wake up early and sleep early. "Night owls" do best if they can sleep until later in the morning.

But when our routines don’t match our biological cycles, the body tries to compensate using responses that evolved to help early humans survive the danger, neuroscientist Foster said. It churns out stress hormones, releases hunger hormones, pumps extra sugar into the blood, and raises blood pressure.

For brief periods, this is not harmful. But when it lasts months or years, we become more vulnerable to cognitive and emotional effects and eventually to cardiovascular diseases, mental illness, diabetes, overweight, and other metabolic disorders, Foster said.

Other studies suggest we also face higher odds of divorce and road accidents.

It’s been estimated that more than half the population in industrialized societies may have circadian rhythms that are out of sync with their schedules. Globally, an estimated one in five workers does regular night shift work, according to a 2020 study by the International Agency for Research on Cancer.

Circadian rhythms shift with age. "Eveningness," or peak alertness at night, tends to be highest in adolescence. We drift toward "morningness" as we age. One consequence of this is that teachers are generally more alert in the morning, but their students are more alert in the afternoon.

A 2016 Rand Corporation report calculated that an employee who works irregular hours commutes 30 to 60 minutes each way, and faces unrealistic time pressures at work sleeps on average about 28.5 minutes per day less than an employee who has regular working hours, commutes no more than 15 minutes one way and is not exposed to unrealistic time pressure and other psychosocial risk factors at work.

This equates to over 173 hours of lost sleep per year.

Ideally, according to Foster and others, society would find ways to mitigate the problems caused by rigid school and work schedules.

What happens in sleep

Sleep itself has cycles, in which the brain and body move through phases, marked by varying brain activity. In the deepest phases of sleep, the brain waves are slowest. The lighter phases have more rapid bursts of activity.

Our most intense dreams usually happen during Rapid Eye Movement (REM) sleep, when brain activity, breathing, heart rate, and blood pressure all increase, the eyes move rapidly, and muscles are limp. Scientists believe dreams in REM and non-REM sleep have different content – the more vivid or bizarre dreams usually happen during REM stages.

During sleep, the brain is extremely active. On a typical night, different regions become engaged as we move through the four main stages of sleep.

If conditions are right, nerve cells in the brain and a cluster of cells in the hypothalamus, the brain’s control center, signal that it’s time to transition to the first stage of sleep. Later, parts of the brainstem will relax our limbs.

Brain waves slow significantly. Eyes move behind closed lids, but muscle activity is minimal. Signals between the nerve cells decline, allowing the brain to process only important information.

During this time, some kinds of memories are consolidated. Memory traces are transferred from a temporary store in the hippocampus to the cortex.

Waste from brain cells’ daytime activity is flushed out in cerebrospinal fluid through a newly identified pathway, called the glymphatic system. It resembles the lymphatic system. It’s formed of star-shaped cells known as astrocytes.

A region in the upper brainstem kickstarts the move into REM sleep. This phase plays an important role in dreaming, emotion processing and brain development.

Brain activity increases, the eyes move rapidly, the heart rate speeds up and breathing becomes irregular.

During REM sleep, the cortex – responsible for cognition and emotion processing – is activated in some regions and deactivated in others. Voluntary muscles in the body are paralyzed, so we do not act out our dreams.

After cycling through non-REM and REM sleep around four to five times, the basal forebrain and other structures receive signals to start exiting sleep.

Light exposure to the optic nerves triggers the release of cortisol and other circadian cues that help start the process of waking up.

When sleep goes wrong

In the U.S. alone, 50 to 70 million people experience some type of chronic sleep disorder, according to the American Academy of Sleep Medicine (AASM).

One billion people worldwide are thought to suffer from obstructive sleep apnea, or OSA, in which the airway gets blocked for short periods, breathing stops, and less air reaches the lungs, increasing the risk for cardiovascular disease. Less common is central sleep apnea, a potentially fatal disorder in which breathing stops and starts unpredictably because the brain isn’t working properly.

About one in three U.S. adults report trouble falling asleep or staying asleep, or waking up too early and not falling back to sleep, according to the American Psychiatric Association. Worldwide, the prevalence of insomnia ranges from 10% to 60%.

Conditions where people oversleep, and fall asleep at inconvenient or dangerous times – such as while driving – are known as hypersomnias. Causes range from a lack of a chemical in the brain that regulates wakefulness to seasonal changes: In winter there is less sunlight sending wake-up signals to the brain. Sleeping too much – more than nine hours per night on a regular basis, according to the AASM – can also be a sign of underlying health problems.

Parasomnias are unwanted events that occur while a person is sleeping. People with parasomnias might move, talk, and express emotions while appearing to be awake, even though they are really asleep.

Among the best known is Restless Legs Syndrome, in which sufferers feel a burning or itching in their legs when they lie down. Others include painful leg cramps in the middle of the night, uncontrollable muscle movements, bruxism, or grinding or clenching of the teeth.

Why enough sleep?

The importance of getting enough sleep is supported by decades of research. The amount we need can vary from person to person, but guidelines from the U.S. National Sleep Foundation recommend seven to nine hours every night for ages 18 to 64 and seven to eight hours per night for older adults.

Sleep deprivation has been shown to cause cognitive impairment akin to that seen when drunk. The longer we are awake, the worse our reaction times, short-term memory, and logical reasoning become, similar to what happens as blood alcohol content (BAC) increases.

For adults, being awake for 17 hours is similar to having a BAC of 0.05%, a level at which driving impairments are seen, according to the U.S. National Institute for Occupational Safety and Health. Some countries use that level as a cutoff for legal intoxication.

One U.S. study of more than 6,800 road accidents found that drivers who slept less than four hours the night before an accident were 15 times more likely to be at fault than drivers who had slept at least seven hours.

Absolute sleep deprivation can be fatal. Inadequate sleep affects everything from the immune system to muscle tone.

Costs of insufficient sleep

The economic costs of poor sleep are enormous.

On an individual level, it leads to reduced productivity, decreased concentration and focus, and increased workplace absenteeism or presenteeism. Sleep-deprived people are also more likely to make mistakes, be less efficient and experience a decline in cognitive abilities, which can affect their performance and prospects.

On a larger scale, poor sleep can also have significant healthcare costs, sending more people to seek healthcare and take medication, and leading to increased medical errors.

Other costs include increases in traffic and industrial accidents, injuries, and decreased workplace productivity and economic competitiveness. A 2019 study found that insufficient sleep reduces voting and other behaviors that are beneficial to society.

Tips for better sleep

Good sleep habits can contribute to better sleep, studies have found. These include going to bed and waking up at roughly the same time every day and making sure your bedroom is quiet, dark, relaxing, and a comfortable temperature. Other routines can also help, such as being physically active during the day and avoiding heavy meals and caffeine before bedtime.

If better habits aren’t enough, other measures include talking therapies such as cognitive behavioral therapy, light therapy, physical appliances to keep the nose and throat clear, surgery, and medication. Lifestyle modifications and weight loss are sometimes recommended as well.

Newer treatments for obstructive sleep apnea include vibrotactile positional therapy, in which a wearable device begins to vibrate when the wearer rolls onto the back; oral pressure therapy which shifts the tongue and soft palate forward, and an implanted upper airway stimulation system.