What is sleep?
Physiologically, sleep is a complex process of restoration and renewal for the body. Scientists still do not have a definitive explanation for why humans have a need for sleep. We do know that sleep is not a passive process or "switching off" of body functions; sleep is believed to be important in many physiologic processes including the processing of experiences and the consolidation of memories. It is also clear that sleep is essential, not only for humans but for almost all animals.
The importance of sleep is underscored by the symptoms experienced by those suffering from sleep problems. People suffering from sleep disorders do not get adequate or restorative sleep, and sleep deprivation is associated with a number of both physical and emotional disturbances.
What causes the body to sleep?
Sleep is influenced by the circadian rhythms (regular body changes in mental and physical characteristics that occur in the course of about 24 hours). These are controlled by brain neurons that respond to light, temperature and hormones and other signals and comprise the body's biological clock. This clock helps regulate the "normal" awake and sleep cycles. Disruption of these cycles can make people sleepy,
or somnolent, at times people want to be awake. For example, travelers experience "jet lag" when they cross time zones. When a New Yorker arrives in Paris at midnight Paris time, his or her body continues to operate (their biological clock) on New York time. It may take several days to reset a person's biologic clock, depending on how much it has been altered by the time change.
Different organ systems in the body recover at different rates.
There is evidence that some aspects of sleep are under genetic influence; a gene termed DEC2 is being investigated as causing people that possess it to require only about 6 hours of sleep. Researchers have only begun to examine the genetics involved in sleep.
Sleep apnea is defined as a reduction or cessation of breathing during sleep. The three types of sleep apnea are central apnea, obstructive apnea, and a mixture of central and obstructive apnea.
- Central sleep apnea is caused by a failure of the brain to activate the muscles of breathing during sleep.
- Obstructive sleep apnea is caused by the collapse of the airway during sleep.
- The complications of obstructive sleep apnea include high blood pressure, strokes, heart disease, automobile accidents, and daytime sleepiness as well as difficulty concentrating, thinking and remembering.
What are the stages of sleep?
There are two general states of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further subdivided (see below).
- REM sleep (rapid-eye movement): REM sleep is unlike any of the other stages of sleep. It was first described in 1953 when sleep researchers noticed a unique pattern of brain waves (signals recorded on an electroencephalogram (EEG), a type of test that measures the electrical impulses within the brain). These brain waves had a fast frequency and low voltage, similar to the brain waves seen in the normal awake state. Other characteristics of REM sleep include complete inactivity of the voluntary muscles in the body, with the exception of the muscles that control eye movements. Rapid eye movements are also observed during REM sleep. People who are awakened during REM sleep often report that they were dreaming at the time. About 20% to 25% of sleep time is REM sleep; in infants it can comprise about 40%.
- NREM (non-rapid eye movement): NREM sleep occurs in 3 stages, according to the pattern of brain electrical activity:
- Stage N1 sleep, or the transition from wakefulness to deeper sleep. This is the lightest stage of sleep, and people may not always perceive they are asleep when in this stage.
- Stage N2 sleep is a true sleep state, and accounts for 40% to 50% of sleep time.
- Stage N3 sleep has been called deep sleep, delta sleep, or slow wave sleep. This stage accounts for about 20% of sleep in young adults.
Disruptions in the entire sleep cycle or in the individual phases are believed to account for the various types of sleep disorders.
How long does it take to get REM sleep?
Sleep typically occurs in cycles that range from 90 to 120 minutes in length, with 4 to 5 cycles occurring during each night's sleep. In the first half of the night, there is a transition from wakefulness into stage N1 sleep, then to stages N2, and N3. Stages N2 and N3 then reappear, followed by the first instance of REM sleep. Cycles of stage N2 and REM sleep alternate with each other for the second half of the night. Typically, there is a greater portion of N sleep in the first half of the night and REM sleep in the later portion of the night.
Why is REM sleep important?
REM sleep makes up less than 25% of total sleep time, and the reason for its importance is not fully understood. Some studies have suggested that REM sleep is necessary for the brain to preserve memories and maintain appropriate neurological connections.
What percentage of sleep should be deep sleep?
Deep (N3) sleep, as defined above, only accounts for about 20% of total sleep. The largest amount of deep sleep takes place in the first half of the night.
How much sleep does a person need?
Individuals vary greatly in their need for sleep; there are no established criteria to determine exactly how much sleep a person needs. Eight hours or more may be necessary for some people, while others may consider this to be too much sleep.
The National Institutes of Health (NIH) suggests that most average adults need about 7 to 9 hours of sleep each night. Newborn babies, by contrast, sleep from 16 to 18 hours a day. Preschool-aged children typically sleep between 10 and 12 hours a day. Older, school-aged children and teens need at least 9 hours of sleep a night. Women in the first trimester of pregnancy have been observed to need a few more hours' sleep than is usual for them.
Does the amount of sleep we need change as we age?
Changes in the sleep cycle do occur with aging. Deep or slow wave sleep (Stage N3) sleep declines as we age, while light sleep (Stage N1) increases with age, so that older adults may spend less time in the more restorative stages of sleep and more time in lighter sleep. Older people are also more easily aroused from sleep. While some people believe that older adults need less sleep as they get older, there is no scientific evidence that older people need less sleep than younger adults.
What are signs and symptoms of sleep deprivation?
Feeling tired or drowsy at any time during the day is one symptom of not having enough sleep. Being able to fall asleep within 5 minutes of lying down in the evening also may be a sign a person may be suffering from sleep deprivation. People who suffer from sleep deprivation often experience so-called "microsleeps," which are short bursts of sleep in an otherwise awake person.
Sleep-deprived people perform poorly on tests such as driving simulators and tests of hand-eye coordination. Sleep deprivation can also magnify the effects of alcohol, meaning that a sleep-deprived person will be more susceptible to becoming impaired after alcohol consumption than a well-rested person. Caffeine and other stimulants cannot successfully overcome the drowsiness associated with sleep deprivation.
What are and what causes sleep disorders?
Sleep disorders are disruptions of the sleep cycle or the quality of sleep. About 50 to 70 million Americans are believed to suffer from chronic sleep disorders, with millions more affected on an occasional basis. Doctors have defined over 70 different types of sleep disorders, but the most common sleep disorders are insomnia, sleep apnea, restless legs syndrome, and narcolepsy.
- Insomnia is the perception of poor-quality sleep, including the inability to fall asleep or stay asleep. Because people differ in their need for sleep, there are no fixed criteria that define insomnia. Insomnia is very common and occurs in 30% of the general population. Approximately 10% of the population may suffer from chronic (long-standing) insomnia. Sleep onset insomnia is characterized as occurring at the beginning of the desired sleep time and lasting for greater than 30 minutes. Sleep maintenance insomnia is when individuals fall asleep, but awaken periodically or for lengthy periods during the night, increasing the wake-after-sleep-onset (WASO).
- Sleep apnea is another common sleep disorder characterized by a reduction or pause of breathing (airflow) during sleep. Central sleep apnea (CSA) occurs when the brain does not send the signal to the muscles to take a breath, and there is no muscular effort to take a breath. Obstructive sleep apnea (OSA) occurs when the brain sends the signal to the muscles and the muscles make an effort to take a breath, but they are unsuccessful because the airway becomes obstructed and prevents an adequate flow of air. Mixed sleep apnea occurs when there is both central sleep apnea and obstructive sleep apnea.
- Restless leg syndrome (RLS), also known as nocturnal myoclonus, is a type of sleep disorder characterized by uncomfortable sensations in the legs and an uncontrollable desire to move the legs. These abnormal sensations usually occur in the lower legs during the evening. Periodic leg movements (PLMs) are related to RLS, but occur after the onset of sleep and are labeled as a sleep disorder or syndrome when the movements cause increased activity in the brain. During the early stages of sleep, these episodes of leg movement often last up to an hour. The abnormal sensations of RLS are quite variable. They have been described as a crawling, creeping, pulling, drawing, tingling, pins and needles, or prickly discomfort. They are not cramping in character. Patients with RLS may have difficulty falling asleep because of the difficulty getting comfortable and an increased urge to move their legs. Many patients with RLS will have PLMs and vice versa, but they are not the same disorder.
- Narcolepsy is a disease of the central nervous system that results uniformly in excessive daytime sleepiness (EDS). Other primary symptoms of narcolepsy include the loss of muscle tone (cataplexy), distorted perceptions (hypnagogic hallucinations), and the inability to move or talk (sleep paralysis). Additional symptoms can include disturbed nocturnal sleep and automatic behaviors (affected persons carry out certain actions without conscious awareness). All of the symptoms of narcolepsy may be present in various combinations and degrees of severity.
Other sleep disorders include:
Sleep Disorders: Insomnia, Sleep Apnea, and More
How are sleep problems diagnosed?
Doctors use a number of different tests to evaluate sleep and determine whether a sleep disorder is present. A careful medical history and physical examination are performed to help identify any medical conditions that may be interfering with the person's sleep. The health care practitioner will also ask about the use of prescription and non-prescription medications as well as alcohol, tobacco, and caffeine use. Laboratory tests may also be used to help diagnose any medical conditions that may cause sleep problems.
In some cases, specialized testing is recommended to help determine whether or not a person may be suffering from a sleep disorder. Some of the most common sleep tests include the following:
- Polysomnography is often simply referred to as a "sleep study." Full sleep studies with additional information about the patterns and events during sleep are most commonly performed in specially designed labs in hospitals or clinics. In this test, functions such as airflow, breathing effort, blood oxygen levels, leg movements, electrocardiogram (ECG), and body position may be measured along with electrodes attached to the face and scalp to measure brain waves (electroencephalogram or EEG) and muscle tone during a night's sleep. Newer technologies have allowed the assessment of sleep disordered breathing in a patient's home setting. This type of out-of-lab testing with a portable sleep monitor usually follows a clinical evaluation by a sleep specialist.
- The multiple sleep latency test (MSLT) is designed to measure daytime sleepiness. The test is based upon the fact that the sleepier an individual is, the faster he or she will fall asleep. In this test, the patient is given four to five opportunities to nap in a quiet, dark room, usually at two hour intervals during the day. Body functions such as EEG and muscle tone are measured as in polysomnography. The time period needed from wakefulness to sleep onset is measured to determine the "sleep latency." This is repeated during each of the naps, and an average time for sleep latency across all the naps is calculated. Usually a sleep latency of 5 minutes or less is signifies severe daytime sleepiness.
- Related to the MSLT is the maintenance of wakefulness test (MWT), which measures the individual's ability to stay awake when reclining in a quiet, darkened room.
- The Epworth sleepiness scale is a questionnaire that is given to patients, often as part of an office visit to a health care practitioner. The test asks individuals to rate how likely they would be to fall asleep in a number of situations (such as a passenger in a car, sitting quietly after lunch, etc.).
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How are sleep problems treated?
The treatment of sleep disorders depends upon the exact disorder and the degree of severity of the symptoms. Both medical and non-medical approaches are generally used in the treatment of sleep disorders. In some cases, such as sleep apnea, surgical treatments may be considered.
In some patients, more than one type of sleep disorder may be present, requiring
a combination of treatment considerations.
Non-medical treatment options are often referred to as sleep hygiene. Sleep hygiene is the practice of behavioral habits that offer the maximum potential for restorative and sound sleep. Good sleep hygiene practices include:
- Avoid caffeine, nicotine, and alcohol use before bedtime. Some studies have shown that caffeine consumed early in the day can have an effect on the ability to fall asleep at night.
- Have and adhere to a regular bedtime and waking schedule.
- Maintain a comfortable sleep environment, including a comfortable temperature.
- Avoid watching television or using electronics with backlit screens in bed and falling asleep with the TV on in
- Do not lie in bed awake, worrying about not sleeping (or anything else negative). This produces anxiety that can actually make the problem worse.
- Get regular daily exercise (it is recommended that individuals avoid exercise two hours prior to bedtime).
Of course, many people with sleep disorders will require treatment beyond sleep hygiene measures. Behavioral therapies are successful for many people who suffer from insomnia. These therapies may consist of stimulus control measures, such as using the bed for sleeping and sex only and not for other activities such as reading or TV watching. Sleep restriction therapies are often used to help individuals avoid staying in bed too long and actually over-sleeping after a night of insomnia.
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Sleep aids (prescription and OTC)
Medications can be of value in treating some types of sleep disorders. However, since sedating medications typically have the potential for addiction and abuse, their use must be carefully supervised by a health care practitioner. Among the types of prescription drugs that have been prescribed for specific sleep disorders include:
- Benzodiazepines, such as triazolam (Halcion), temazepam (Restoril), and lorazepam (Ativan)
- Newer, non-benzodiazepine sedative drugs: zaleplon (Sonata), zolpidem (Ambien or Ambien CR, Zolpimist), and eszopiclone (Lunesta).
- Ramelteon (Rozerem), an insomnia drug that acts by mimicking the action of melatonin (see below)
- Suvorexant (Belsomra) is the first in a new class of drugs known as orexin receptor antagonists (ORAs for the treatment of insomnia. Suvorexant works by promoting the natural transition from wakefulness to sleep by inhibiting the wakefulness-promoting orexin neurons of the arousal system.
- Antidepressant medications have been used to treat insomnia in people who may also suffer from depression. Examples are trazodone (Desyrel), amitriptyline (Elavil, Endep), and doxepin (Sinequan, Adapin).
- A number of prescription drugs have been used to treat restless legs syndrome, including carbidopa-levodopa, opioids (such as propoxyphene [Darvon, Darvon-N, Dolene]) or tramadol (Ultram) for intermittent symptoms, carbamazepine (Tegretol, Tegretol XR , Equetro, Carbatrol),clonazepam (Klonopin), diazepam (Valium, Diastat), triazolam (Halcion), temazepam (Restoril), baclofen, bromocriptine, clonidine (Catapres, Catapres-TTS, Jenloga), gabapentin (Neurontin), ropinirole (Requip) and pramipexole (Mirapex).
- In sleep apnea and other sleep disorders in which airway obstruction is a problem, topical nasal decongestants may provide some relief. However, many clinicians warn people with sleep apnea never to use sleeping pills or medications that are sedative as the person could be prevented from waking enough to stimulate breathing, which could lead to brain damage or sudden death.
OTC sleep medications are sometimes used for the short-term treatment of insomnia. These include the sedating antihistamines such as diphenhydramine (Benadryl). However, this is not a recommended use of these or other similar drugs due to their many side effects and the possibility of long-term drowsiness the following day.
Melatonin, a chemical released from the brain which induces sleep, has been tried in supplement form and promoted as a natural sleep remedy for treatment of insomnia. But studies have shown that it has been generally ineffective in treating common types of insomnia, except in specific situations in patients with known low levels of melatonin.
CPAP devices (continuous positive airway pressure; a device worn over the face that holds the airway open by maintaining constant air pressure) or AutoPAP (PAP delivered over a range of pressures) and dental appliances have been effective in the management of sleep-related breathing disorders, including sleep apnea. CPAP is typically the first line of therapy for most adult patients with obstructive sleep apnea. Surgery can be effective for some patients and may help patients respond to CPAP. Newer, implantable devices that stimulate the muscles of the upper airway during sleep may also be a treatment option for some patients.
Why do we sleep?
Are sleep problems and disease related?
Sleep problems occur in a number of different medical and psychiatric conditions. For example, asthma attacks and stroke are conditions that tend to occur frequently during the night or early morning hours. The relationship between sleep stages and certain types of epileptic seizures is complex and not completely understood, but certain sleep stages tend to either exacerbate or prevent the spread of seizure activity in the brain.
Sleep problems occur with chronic pain and conditions in which pain is worse at night, because the pain may interfere with sleep. Pain medications and other types of medications taken on a regular basis for chronic conditions can also have an effect on an individual's sleep pattern. Those suffering from cancer, Alzheimer's disease, and brain injury are also commonly affected by sleep disturbances.
Psychiatric diseases such as depression are also associated with sleep problems. This condition can be associated with both too much sleep and too little sleep. In fact, sleep problems are associated with a majority of mental disorders, and poor quality or insufficient sleep can worsen the symptoms of mental or psychiatric conditions.
How can I get a good night's sleep?
Practicing good sleep hygiene (see above), including maintenance of a regular bedtime and awakening schedule, is the best way to ensure restful and restorative sleep. Avoidance of caffeine, alcohol, nicotine, and strenuous exercise in the hours prior to bedtime can also help improve the quality of your sleep. Many people report that they lie awake at night worrying about problems or situations they will face during the coming day. In this case, it can be helpful to write a to-do list or a list of items to act upon the following day prior to bedtime, giving yourself permission to "let go" of these items during the night.
If you are concerned about the quality of your sleep or if you have the symptoms of a sleep disorder, it is important to consult your health care practitioner. He or she can help you determine the cause of your sleep problem and recommend appropriate therapy.
Medically Reviewed on 9/4/2018
Medically reviewed by Jon Glass, MD; Board Certification: Neurology
Iber, C, Ancoli-Israel, S, Chesson, A, Quan, SF. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specification, 1st ed, American Academy of Sleep Medicine, Westchester, Illinois 2007.
He, Y, Jones, CR, Fujiki, N, et al., The transcriptional repressor DEC2 regulates sleep length in mammals, Science, 325: 866–70, 2009
Tononi, G. and C. Cirelli. "Perchance to prune. During sleep, the brain weakens the connections among nerve cells, apparently conserving energy and, paradoxically, aiding memory." Scientific American 309.2 (2013): 34-39.
NIH.gov. Brain basics: Understanding sleep.