Doctors are increasingly waking up to the idea that getting enough sleep is crucial. In 2022, the American Heart Association advised physicians that asking patients how many hours they sleep is as important as checking their blood pressure, smoking status, diet and exercise habits.
But many of us routinely force our bodies to fight sleep. We drink caffeinated beverages by day to stay alert and alcohol to wind down at night. 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.
We have clocks in our brains and cells
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 UK.
The circadian clock
The body’s circadian rhythm runs about 24 hours. Primarily regulated by light and darkness, the cycle affects various parts of the body differently.
In humans, here’s how a typical day looks.
The circadian clock keeps our organs operating at peak performance at the right time of day. Certain organs, like the kidneys, function best during the day, whereas nighttime is most important for the brain.
Kidneys and bladder
Waste filtration is highest during the day and lowest at night. Kidney hormones that help regulate blood pressure and red blood cell production also follow a daily rhythm.
The cell activities that are important for physical performance, muscle growth and repair peak during the day and dip at night.
In the evening the cells focus on storing fat. In the morning the emphasis is on breaking down fat and producing anti-inflammatory hormones.
The skin’s ability to regenerate and respond to stress follows a circadian rhythm. Injuries incurred at night heal slower than those you get during the day.
These produce hormones that are critical in regulating the body’s stress responses and other important chemical processes. Disruptions to their circadian rhythm can upset that process.
The brain releases the hormone melatonin to help regulate the sleep-wake cycle. Inside it, the hypothalamus and pituitary gland help raise stress hormone levels in the morning to boost alertness; they also help stimulate glucose production, providing energy for the day.
Different functions peak at night and in daytime, so disruptions to the circadian rhythm impair the immune response. Doctors recently linked long COVID with circadian rhythm sleep/wake disorders.
Blood pressure, heart rate and breathing rate are highest during the day and lowest at night. Our production of red blood cells and hemoglobin, the protein that carries oxygen, also peaks in the daytime.
Release of digestive enzymes is highest by day. To keep blood sugar from getting too high, insulin production in the pancreas peaks in the morning. At night, the pancreas releases more glucagon so blood sugar does not fall too low. Our muscles’ work moving food through the intestines, and the liver’s detoxification of harmful substances, also follow diurnal patterns.
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 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 for 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.
All creatures need sleep
Researchers have recently discovered that fossilized plants living more than 250 million years ago – just as dinosaurs were starting to appear – already had circadian rhythms that drove them to fold their leaves at night and reopen them during the day.
All animals sleep, but it can look widely different depending on the species. According to a Boston University database of the sleep characteristics of 133 mammalian species, red kangaroos sleep for just under two hours in a 24-hour period, while armadillos and bats can sleep for nearly 20 hours. Some animals, including dolphins and whales, sleep unihemispherically – one half of the brain sleeps at a time to allow them to be constantly alert.
Hours of sleep per day
Sleeps for less than
2 hours a day
South American Armadillo
Sleeps more than
20 hours a day
Hollow dots show species that sleep with one half of the brain resting while the other remains alert
Dolphins need to breathe the air on the surface, so can’t fall into deep sleep like humans because they would drown
Right hemisphere sleeping
Left hemisphere awake
Unihemispheric sleep allows dolphins to control their breath and be aware of predators
Each eye is connecting to
opposite brain hemisphere
Left eye closed
Right eye open and alert
After two to four hours dolphin swaps the sleeping side
What happens when we 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. In a typical night, different regions become engaged as we move through the four main stages of sleep.
Behind some internal structures at the center of the brain, we can see deeper regions. This is where the sleep cycle begins.
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.
Eye movement and muscle activity slow.
Sinking into sleep
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.
Moving into REM sleep
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 4 to 5 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.
This chart shows how we cycle through sleep stages. We generally go from being awake to sleeping lightly in non-REM stage 1, to sleeping increasingly deeply in two further stages. Then we transition back to lighter sleep, into REM sleep and back down again, and so on until we wake up.
A typical night’s sleep
Non-REM stage 1
Non-REM stage 2
Non-REM stage 3
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, and bruxism, or grinding or clenching of the teeth.
What happens if we don’t sleep enough?
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 7 to 9 hours every night for ages 18 to 64 and 7 to 8 hours per night for older adults.
Sleep per 24 hours
Seven or more hours
65 and older
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.
People with obstructive sleep apnea are at higher risk for high blood pressure, strokes, clogged heart arteries, and irregular heartbeats.
Irregular sleep schedules and poor sleep quality are associated with poor blood sugar control in people with diabetes or prediabetes.
Studies in humans and animals have found links between too-little sleep and excess weight.
Sleep disorders are a hallmark of post-traumatic stress disorder, schizophrenia, bipolar disorder and other mental illnesses. Lack of sleep also increases risks for depression, anxiety, and even suicidal thoughts.
Vaccines induce fewer antibodies when they are given to sleep-deprived individuals, new research shows.
Sleep cycle disruptions can impact fertility in men and women.
Sleep deprivation reduces protein synthesis, which decreases the body’s ability to repair muscle damage and increases the harmful effect of high-intensity exercise.
The 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 health care 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.
The Rand Corporation researchers calculated in 2016 that the U.S. loses an equivalent of about 1.23 million working days annually due to insufficient sleep, corresponding to about 9.9 million working hours. Annual losses for other countries were estimated to average 0.6 million working days in Japan and 0.2 million days each in the UK and in Germany.
The economic impact
Details from a RAND Corporation report published in 2016.
in GPD terms
The Rand Corporation economists estimated that the U.S. was sustaining an annual economic loss related to insufficient sleep of $280 billion to $411 billion, or up to 2.28% of the overall economy. Japan, they estimated, was losing up to almost 3% of its economy to insufficient sleep, followed by the UK and Germany.
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, caffeine and alcohol before bedtime.
Establish a regular bedtime and wake-up time, even on weekends.
Avoid sleep under 12.5°C and more than 21°C.
Remove screen technology from the bedroom.
Reduce caffeine and alcohol intake. Avoid eating for 3-2 hours before going to bed.
Remove visible clockfaces from view in the bedroom, preventing clock-watching anxiety at night.
Avoid exercise for several hours before sleep.
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 vibro-tactile positional therapy, in which a wearable device begins to vibrate when the wearer rolls onto the back; oral pressure therapy that shifts the tongue and soft palate forward, and an implanted upper airway stimulation system.
In the United States, a list of board-certified sleep medicine physicians and accredited sleep disorders centers is available from the American Academy of Sleep Medicine.
Additional work and development by
Phylogeny of Sleep, Boston University; RAND Corporation; Circadian Mechanisms in Medicine, Ravi Allada and Joseph Bass; Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives, Gian Gastone Mascetti.