Sleep is as essential to our daily needs as food and water. Although we may feel that sleep simply rests our tired bodies, our brain remains active throughout the night. Sleep plays a critical role in brain as well as physical functioning.
What Happens When We Sleep?
Our internal body clock, called a circadian clock, tells us when we are ready to sleep. There are actually several circadian clocks in the body, found in the brain and other organs. They are triggered by cues such as daylight (we feel alert) and darkness (we feel drowsy). These clocks can also be triggered by artificial bright light or stimulants like caffeine and alcohol that cause us to feel awake even if it is nighttime.
There are several phases of sleep our body experiences. They are classified as REM (rapid eye movement) and non-REM sleep. We cycle repeatedly through these phases about 4-6 times throughout the night, and it is not uncommon to wake up briefly between cycles.
Stage 1. You transition from being awake to a restful state.
Stage 2. You are in a light sleep state. Your breathing, heart rate, and muscle movements slow down. Brain activity also slows, and your body temperature drops.
Stage 3. You are in a deep sleep state. This stage often occurs early in the sleep cycle immediately following light sleep. Your heart rate and breathing are the slowest during this phase, and you are not easily awakened. Events of the day are processed and stored in your memory. A lack of deep sleep can leave one feeling tired in the morning even if achieving an adequate duration of sleep.
During REM, your pupils twitch and move quickly from side to side underneath closed eyelids. Brain activity rises as you breathe faster and your heart rate increases. It is the phase of sleep when dreams are most common, and certain nerves signal your limbs to become temporarily paralyzed so you do not act out the dream. REM tends to occur later at night and into early morning. Memory is processed and stored during REM sleep.
Why do we dream?
Hormones that Regulate Sleep Cycles
There are various neurotransmitters and hormones released by the brain that send signals to promote sleep or wakefulness.  Many of these chemicals are stimulated by light or darkness.
- GABA is a neurotransmitter that decreases nerve cell activity, playing a major role in allowing the body to sleep.
- Adenosine is another neurotransmitter that gradually accumulates in the brain during the day, and at high concentrations makes us sleepy at night. Caffeine in coffee and other beverages can keep us awake as it blocks brain receptors for adenosine.
- Melatonin is a hormone released by the brain when it is dark. It travels to cells to tell the body to sleep. Sunlight or exposure to light inhibits the production of melatonin and increases the release of cortisol, which awakens us. If we are exposed to too much artificial light (such as the blue light emitted from smartphones or televisions) late at night, less melatonin may be released making it harder to fall asleep.
- Serotonin, the body’s “feel-good” chemical, is a neurotransmitter associated with both sleep and being awake. The brain releases this chemical during daylight but also uses it to form melatonin at night.
- Hormones that counteract sleep include norepinephrine, adrenaline, histamine, and cortisol. These are secreted in response to stress and cause the body to be awake and alert. If one experiences prolonged or chronic stress, the body releases adrenocorticotropic hormone (ACTH), which in turn releases cortisol. Levels of ACTH tend to be higher in people who have insomnia.
Immediate Effects of Sleep Deprivation
About one-third of American adults do not get enough sleep each night, according to the Centers for Disease Control and Prevention. [2,3] Short sleep duration in adults is defined as less than 7 hours of sleep in 24 hours. About 40% of adults report unintentionally falling asleep during the day at least once a month, and up to 70 million Americans have chronic sleep problems. Because of the public health burden of poor sleep health, achieving sufficient sleep in children and adults was included as a goal in the Healthy People 2020 goals. 
Sleep helps to process your thoughts from the day as well as store memories, so a lack of good-quality sleep can lead to difficulty focusing and thinking clearly. You may feel tired, irritable, or anxious during the day. Performance at work or school may suffer. Your reaction time may be slowed, increasing the risk of driving accidents.
In children, insufficient sleep can lead to attention and behavior problems or hyperactivity. In the elderly, lack of sleep may decrease focus and attention, leading to a greater risk of falls, bone fractures, and car accidents.
There are several reasons people may get insufficient sleep:
- Poor sleep habits (watching television or using screens late at night, drinking caffeinated or alcoholic beverages at night, not following a regular sleep schedule).
- Your sleep environment is too noisy, too light or otherwise not conducive to sleep.
- You attempt to sleep outside of the body’s natural circadian clock (working an overnight shift and trying to make up for sleep during the day).
- You have a sleep disorder, such as sleep apnea, insomnia, or periodic limb movements that reduces deep or REM sleep or causes frequent awakenings.
- You have a medical condition such as heart, lung or kidney disease, or chronic pain, which causes frequent awakenings.
Sleep Deficiency and Disease Risk
If you experience continued sleep deprivation, you will develop a condition called sleep deficiency. This is a state in which you cannot make up the many lost hours of sleep. Sleep deficiency increases the risk of obesity, diabetes, cardiovascular disease, depression, and even early death.
Several studies show that sleep deprivation (i.e., regularly less than 7 hours of sleep a night) is a risk factor for obesity. A Nurses’ Health Study found an association between those who slept the least (5 hours or less a night) and having the highest BMI and greatest weight gain.  One reason may be a disruption in appetite hormones that regulate feelings of hunger (called ghrelin) versus satisfaction (called leptin). Ghrelin levels rise while leptin levels drop with lack of sleep; this can cause higher calories to be consumed due to experiencing strong hunger at the same time that one feels less satiated after eating. A preference for foods high in fat and carbohydrate has been observed. [6,7] The risk of hunger also increases simply by being awake longer, which prolongs the time from the last meal eaten to bedtime.  Insufficient sleep also can trigger the “reward” areas in your brain to crave high fat, high caloric foods. 
One may think that getting less sleep would mean more activity due to being awake longer and therefore using more calories. However, studies have found either no increase or very small increases in energy expenditure with sleep deprivation, and even a tendency towards reduced physical activity due to fatigue.  Less physical activity combined with the increased calorie intake associated with sleep deprivation increases the risk of obesity.
Other effects of poor sleep include increased fat storage in the belly area, higher body mass index, poorer quality diet, and decreased insulin sensitivity. [6,10] Interestingly, some studies have also shown that longer sleep times (more than 9 hours) are also associated with developing belly fat compared with sleeping 7-8 hours a night. 
Epidemiological and laboratory studies show a higher risk of diabetes mellitus with both too little sleep (less than 7 hours) and longer sleep durations (more than 9 hours). Metabolic changes may occur with chronic insufficient sleep, such as higher cortisol levels leading to increased blood glucose. Clinical studies have found both increased glucose and insulin levels (suggesting insulin resistance) and reduced insulin sensitivity in sleep-deprived individuals.  Disruption in the regulation of appetite hormones as seen with higher ghrelin and lower leptin levels may lead to increased food intake and weight gain, also increasing the risk of insulin resistance. 
Some people who have insufficient sleep have a condition called obstructive sleep apnea, which blocks breathing in the upper airway tubes, often because of increased fat in the tongue. Sleep apnea is independently associated with insulin resistance; a lack of oxygen while sleeping can cause oxidative stress and inflammation that are believed to progress toward insulin resistance. 
Both shorter and longer sleep durations are associated with cardiovascular diseases. [12,13] Proposed reasons include activation of the sympathetic nervous system and impaired endothelial function, which can lead to elevated blood pressure and hardening of arteries. There may also be greater release of pro-inflammatory cells and decreased immune function. Metabolic changes include a disruption in appetite hormones and circadian rhythms that lead to inflammatory conditions. 
- People sleeping less than 6 hours a night, particularly women, were 20-32% more likely to develop hypertension compared with those sleeping 7-8 hours a night.  Having 5 or less hours of sleep a night was associated with double the risk of developing hypertension in another study. 
- In a Nurses’ Health Study, the risk of heart disease was almost 1.4 times higher in women sleeping 5 hours or less a night and 9 hours or more a night compared with those sleeping 8 hours a night. 
- In postmenopausal women, those who slept 5 hours or less or 10 hours or more a night had a 25% and 45% increased risk of heart disease, respectively. 
- Individuals with inadequate sleep as well as long sleep durations showed an increased risk of strokes. 
- Individuals with poor sleep quality or insomnia symptoms are 40% more likely to develop hypertension 
- People with obstructive sleep apnea are at increased risk for stroke, heart attack, and heart failure.
Poor sleep and insomnia (an inability to sleep or stay asleep) are associated with depression, especially if the insomnia becomes chronic.  Insomnia is also associated with increased likelihood of insomnia relapsing over time. Poor sleep quality can impair functioning, increase fatigue, and lead to mood changes. The reverse is also true in which depressive symptoms of intense sadness or hopelessness can interrupt sleep. Insomnia as well as oversleeping are common signs of clinical depression, according to the National Institute of Mental Health.  Treating the depression may lead to improvements in sleep quality. If there is an underlying medical disorder causing the insomnia such as obstructive sleep apnea or chronic pain, then treatment should address these first.
In 2021, authors of a cohort study sought to tease out the association of early brain changes and sleep changes by including younger patients 50 years of age. They followed 7,959 participants for up to 25 years and found that participants who were between the ages of 50-70 years and slept 6 hours or less a night showed a 30% higher risk of developing dementia in later life, compared with those who slept 7 hours.  The association was only slightly weaker when authors controlled for various factors independently associated with dementia like cardiometabolic status (high blood pressure, diabetes mellitus, body mass index, cardiovascular disease), sociodemographic variables (age, sex, ethnicity, education, marital status), health behaviors (smoking, alcohol, exercise, intake of fruits and vegetables), and mental health factors (depression). The authors did not find an association with longer sleep durations (8 or more hours) and dementia, though this may have been due to the low number of participants who slept longer durations.
Prospective cohort studies have found that both a chronic lack of sleep (less than 7 hours) and long sleep durations (more than 8 hours) are associated with greater risk of death from all causes. [7,13] Obstructive sleep apnea and insomnia are also associated with increased mortality. [21,22] However, long sleep durations appear to be more associated with increased risk of mortality than inadequate sleep.  Some studies show that women may have greater risk of mortality related to short sleep durations than men.  Longer sleep times are associated with several factors that are associated with mortality, including fatigue, stress, obstructive sleep apnea, and increased inflammation of heart arteries. More research with randomized controlled trials is needed to better understand the reasons for these findings.
Medical Conditions that Interfere with Sleep
- Obstructive sleep apnea (OSA)—Symptoms of OSA include snoring or gasping for air that causes interruptions in sleep and prevention of good-quality sleep. Sleep apnea also causes oxygen levels to drop during sleep, which can pose a stress on the heart, brain and other organs. People with OSA may not be aware that they are awakening frequently in the night, but do not get refreshed sleep, feeling excessively sleepy or tired during the day. Continuous positive airway pressure (CPAP) devices may be prescribed, which provides pressurized air to the nose and throat, preventing the upper airway from collapsing. Another common treatment is dental devices that move the jaw forward and increasing the airway size. Obesity is a risk factor for OSA because carrying extra weight, particularly in the neck area, can contribute to obstructed breathing passages. About 70% of adults with OSA have obesity, and a significant improvement in OSA is seen with weight reduction.  OSA is a risk factor for insulin resistance, hypertension, type 2 diabetes, cardiovascular disease, and early mortality. 
- Restless leg syndrome—This condition is associated with discomfort in the legs accompanied by an urge to move, which disrupts sleep. It is believed that abnormal levels of the neurotransmitter dopamine may be responsible, so medications are given to correct this. In some cases, low levels of iron can result in this disorder.
- Insomnia—This condition is defined as the inability to sleep or stay asleep. An individual may have a hard time falling asleep, or may sleep but then awaken in the early morning and be unable to return to sleep. Short-term insomnia can be caused by stress or traumatic events (divorce, job loss, death of a loved one). Chronic or long-term insomnia may be caused by ongoing anxiety, working different work shifts that disrupt the body’s circadian rhythms, poor sleep habits, medical conditions that can interrupt sleep (chronic pain, gastroesophageal reflux disease), or medications that have a stimulating effect. Insomnia often can be treated with behavioral therapies, although sometimes sleep medications are prescribed.
- Genetic—Studies have found specific gene variants that are associated with insomnia. [24,25] The same genes for insomnia were also associated with higher levels of body fat, depression, and heart disease. Research has also found that sleep apnea clusters within families, and genes have been identified that appear to increase risk for sleep apnea as well as cardiovascular disease.  More research is needed in this area.
What if I work the night shift?
People who work overnight shifts, early morning shifts, or rotating shifts (both day shifts and night shifts) may develop something called shift work disorder (SWD). Studies show that those with SWD have poorer sleep quality than day workers.  They may take longer to fall asleep, experience insomnia, and feel excessive sleepiness while awake. This is caused by attempts to sleep in daylight, which opposes natural circadian rhythms. Poor sleep hygiene further aggravates the problem. SWD is associated with decreased alertness, higher risk of work-related accidents, and increased depression and anxiety. SWD is also associated with metabolic changes increasing the risk of heart disease, obesity, and digestive problems caused by irregular eating habits or poor diet. The following tips can help if you work nontraditional hours: 
- Request to work the same shift several nights in a row, to avoid flipping between day and night shift schedules on consecutive days. This helps to regulate the circadian system.
- Commit to a consistent sleep schedule, darkening the bedroom with blackout shades, and creating a quiet atmosphere as much as possible. You might reduce light exposure even earlier by wearing sunglasses as soon as you leave work. To reduce noise, wear earplugs and use a white noise machine to block sounds.
- After finishing a night shift, try to return home and go to bed as soon as possible. Running errands, watching television, talking with family, or exercising can re-energize your body so that falling asleep becomes more difficult.
- Although it is tempting to run errands and attend medical appointments during the day when places are less crowded, try to minimize doing them immediately after work so that you can return home and honor your sleep schedule.
- Try to keep a set meal schedule. Do meal planning to ensure that quick easy meals are ready when you arrive home, and bring prepped meals/snacks to work for overnight shifts to prevent reliance on fast food and takeout meals. Try to avoid eating a large meal right before bed, which can increase the risk of reflux and indigestion.
Sleep Deficiency and Eating Behaviors
Epidemiological studies show that insufficient sleep is independently associated with a higher risk of obesity. Clinical studies of of sleep-restricted adults show an increased hunger and calorie intake when participants are allowed free access to food.  A preference for late evening or nighttime food intake and increased snacking has been observed.  There also appears to be a food preference for higher carbohydrate and fat foods, which could partly explain the overall higher calorie intake.
Changes in hormone levels that signal either hunger or satiety have also been observed in clinical sleep restriction studies. Leptin is a hormone associated with satisfaction. When food enters the stomach, leptin is released from fat cells and travels to the brain where it signals the body to stop eating by creating a sensation of fullness. People with obesity may actually have very high levels of leptin; the more body fat one has, the more leptin is produced in fat cells. However, a condition called leptin resistance may occur in which the brain does not receive the usual signal from leptin to stop eating. In response, more and more leptin is released. Lower leptin levels as well as high leptin levels suggesting leptin resistance have been observed in sleep-deprived adults. 
Ghrelin, the “hunger hormone,” typically has the opposing action of leptin. It is released in the gut and sends hunger signals to the brain when someone is not eating enough. About three hours after eating a meal, ghrelin levels drop. Clinical studies have found that sleep restriction leads to elevated ghrelin levels. 
Despite this interesting theory of poor sleep leading to changes in appetite hormone levels, other studies have found no changes and therefore the association is still inconclusive.  Conflicting findings may be due to differences in the study participants (e.g., age, gender) and differences in how the researchers defined the duration and severity of sleep restriction.
How Much Sleep Do We Need?
Sleep needs change as we age, with the average person generally requiring less sleep at older ages. However, specific sleep amounts vary by individual. According to the National Sleep Foundation and American Academy of Sleep Medicine (AASM), newborns need the most sleep, at 14-17 hours a day, followed by infants at 12-16 hours a day including naps. Toddlers need about 10-14 hours a day. Preteens and teenagers need about 8-12 hours a night, and adults about 7-8 hours a day.  A consensus by the AASM and Sleep Research Society recommends that adults should sleep 7 or more hours a night to promote optimal health. 
Despite these general recommendations on sleep duration, individual differences in sleep requirements exist. In most epidemiologic studies, increased risk of adverse health outcomes such as obesity, diabetes, and cardiovascular disease, has been observed among those who reported sleeping 5 hours or less per day, and 9 hours or more per day. Thus, a range of sleep hours (more than 5 and less than 9) is considered appropriate for most healthy adults.
Other factors such as quality of sleep are important, because just meeting the total recommended sleep hours may not be enough if one wakes up frequently in the night. A common belief is that lost sleep from a late night out or studying can be recovered by “sleeping in” another day or taking naps. However, both of these methods disrupt the body’s circadian rhythms and may deprive the body of deeper sleep stages. In fact, increased variability in how much sleep we get from night to night is associated with an increased risk of developing metabolic and heart diseases.  It is important to respond, whenever possible, to the body’s natural signals of sleepiness.
What about supplements, medicines, and other therapies for sleep?
Two popular herbal supplements, melatonin and valerian, are used as sleep aids. Melatonin has been shown to quicken time to sleep and have modest benefits on sleep duration and quality, but can cause daytime drowsiness. It is well tolerated in adults with few reported adverse events in doses up to 10 mg. The American Academy of Sleep Medicine (AASM) recommends the judicious use of melatonin for certain sleep and circadian disorders such as shift work disorder or jet lag. 
Valerian contains small amounts of GABA, a sleep-promoting neurotransmitter, and some studies have shown that valerian can improve sleep. However, other studies have found no difference in sleep when taking valerian compared with placebo, and there appears to be minimal benefit in those who have diagnosed insomnia. The AASM does not recommend valerian for insomnia disorder. 
It is important to note that supplements are not reviewed by the U.S. Food and Drug Administration for safety or effectiveness. Therefore doses and preparations of these herbs can vary widely. A study of 31 melatonin products found that the melatonin levels in the pills ranged between 83%-478% of the dose reported on the label.  More than 70% of the products varied from the labeled dose by more than 10%. If supplements are used, look for a label verifying its quality from a third-party, such as from the U.S. Pharmacopeia.
Common medicines prescribed for sleep include sedatives such as benzodiazepines (e.g., Valium, Xanax, Klonopin, Ativan*). They help with falling asleep initially, but tend to reduce the amount of deeper sleep. They are not recommended for long-term use because they can worsen insomnia, increase depression, and impair memory, and are associated with increased risk of falls, cancer, and early death.  Long-term use of benzodiazepines can promote psychological dependence, and there is a risk of addiction and abuse.  Tolerance can also develop over time, requiring larger doses to maintain their effectiveness. Because of these side effects, benzodiazepines are not recommended to treat insomnia in older adults.  There are other classes of sleep medications including non-benzodiazepines (e.g., Lunesta, Ambien) and antidepressants (e.g., Zoloft) that also quicken the time to fall asleep but may interfere with deeper sleep stages. Anticholinergic medications (e.g., Benadryl) can increase the risk for cognitive impairment and decline. Generally, sleep medicines are most effective when used occasionally or for a short time of less than one month. The American Academy of Sleep Medicine (AASM) recommends that cognitive behavioral therapy be used as the initial treatment for insomnia. [*The inclusion of brand names is included for reference and does not constitute an endorsement. The Nutrition Source does not endorse any specific brands.]
Randomized clinical trials have shown that cognitive behavioral therapies (CBT) for sleep such as minimizing napping during the day, relaxation training, breathing exercises, and sleep hygiene are highly effective and recommended as first-line treatments for insomnia. [34,32] They have been found more effective than medications for the long-term management of insomnia. People may be asked to keep a sleep journal to record sleep habits and activities performed around bedtime, which can help determine the most appropriate CBT.
Sleep Hygiene Tips
- Set a sleep schedule and stick to it. Try to go to bed at night and awaken in the morning around the same times, even on weekends. This helps to regulate the body’s sleep cycles and circadian rhythms.
- Try to exercise at some point in the day but avoid vigorous activity (running, fast dancing, high-intensity interval training or HIIT) one hour before bedtime. Regular exercise of adequate intensity can promote muscle relaxation and deeper sleep later on.
- Try to avoid large meals, heavy snacking, or alcohol 2-3 hours before bed.
- If you are sensitive to caffeine, try to avoid drinking caffeinated beverages 4-6 hours before bedtime.
- Stop using electronic devices an hour before bed, especially those emitting blue light such as smartphones, tablets, and televisions.
- Schedule before-bed activities to signal that you are winding down, such as changing into pajamas and brushing teeth.
- Create a quiet, dark, relaxing environment in your bedroom. Dim the lights and turn off your cell phone’s sound and vibration modes if possible.
- Ensure a comfortable temperature, as feeling too hot or cold can disrupt sleep.
- Create calming bedtime rituals such as practicing deep breathing exercises, doing light yoga stretches, or listening to soothing relaxing music. Many meditation podcasts, apps, and YouTube videos offer these tools for free.
- If you awaken and can’t return to sleep, don’t stay in bed. Get up and do quiet relaxing activities, such as reading, until you feel tired enough to fall back asleep.
Does exercising at night disrupt sleep?
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