Sleep and Wakefulness
Sleep is a normal state of rest that is characterized by unconsciousness, reduced activity, and limited sensory responsiveness. Sleep differs from other states of reduced consciousness, such as drug intoxication or coma, because it is spontaneous, periodic, and readily reversible. Sleep is usually described by contrasting it with wakefulness, which is characterized by consciousness, sensory responsiveness, and purposeful activity.
Sleep is one of the least understood aspects of human and animal behavior. It occurs in virtually every vertebrate species and seems to be necessary to healthy functioning, but science has been slow to discover how and why sleep occurs. The biological events that take place during sleep are subtle, and many seem to occur at a cellular level within the brain. These events are difficult to observe, and as a consequence our understanding of sleep has developed slowly.
Stages of sleep
Although the sleeping person seems inactive, the sleeping brain exhibits variations in activity throughout the sleep period. Recordings of brain activity, known as electroencephalograms (EEGs), show patterns that occur in a regular cycle lasting about 90 to 100 minutes. This cycle includes relatively brief periods of rapid-eye-movement
(REM) sleep, characterized by back-and-forth movement of the eyes and changes in autonomic nervous system activity. REM is absent in the other phases of the sleep cycle, which are characterized as non-REM (NREM) sleep. Sleep can be divided into five distinct stages based on EEG and REM activity:
- Stage 1 NREM sleep: This lightest stage of sleep occurs as the person is just falling asleep. Stage 1 accounts for about 5% of a normal sleep period.
- Stage 2 NREM sleep: During this period the EEG exhibits characteristic patterns known as "sleep spindles" and K-complexes. This stage accounts for about 50% of a normal sleep period.
- Stage 3 NREM sleep: This stage is characterized by "slow wave" EEG activity, which is associated with deep sleep.
- Stage 4 NREM sleep This stage is very similar to Stage 3, the only difference being the amount of slow wave sleep that occurs. Together, Stages 3 and 4 account for about 20% of a normal sleep period.
- REM sleep. The EEG pattern of this stage is similar to that of Stage 1 NREM sleep. The sleeping person exhibits rapid eye movements and autonomic changes, as well as inactivity of the skeletal muscles. Most dreaming occurs during this stage of sleep. This stage accounts for about 20 to 25% of a normal sleep period.
The first 90-minute sleep cycle of the night begins with Stage 1 NREM sleep and progresses through Stages 2, 3, and 4, ending with a period of REM sleep. Subsequent cycles usually replace Stage 1 with the REM period. In a typical night of sleep, the earlier cycles tend to include more Stage 3 and 4 NREM sleep, with briefer REM periods. As the night progresses, the REM periods tend to get longer while the NREM periods get shorter.
Sleep and biological rhythms
Sleep is one of several biological processes that exhibit a pattern known as a circadian rhythm. A circadian rhythm recurs spontaneously on about a 24-hour cycle. Humans tend to sleep and wake up according to internal circadian rhythms, which seem to be part of our self-regulatory systems.
Circadian rhythms are regulated by a structure in the brain called the superchiasmatic nucleus, which is influenced by exposure to light. Damage to the superchiasmatic nucleus may result in loss of circadian rhythms, however, the individual still exhibits periodic tendencies to fall asleep. This is because a second, homeostatic process also regulates sleep. The individual seems to need sleep after periods of being awake, and the longer the period of wakefulness, the greater the likelihood that the person will fall asleep.
Sleep and the life cycle
The duration and patterning of sleep shows developmental changes throughout the life cycle. Newborns tend to sleep about 16 hours each day, with sleep occurring in relatively brief two to four-hour periods. As children grow, they sleep for longer periods at a time, with fewer sleep periods in a day, until they achieve the adult pattern of a single sleep period each day. The total amount of sleep also declines during childhood, until reaching the adult average of seven to nine hours per night.
In most adults, the amount of nightly sleep remains fairly stable until old age. Adults over 65 years of age tend to sleep less and report more frequent awakenings than younger adults. More than half of adults over 65 report some difficulty with sleep, although these sleep disturbances are often related to other problems, such as poor health or depression.
The patterns of REM and NREM sleep also show developmental changes. REM sleep tends to be much more prevalent in infants, with as much as 50% of their sleep time taken up by REM activity. This percentage declines throughout childhood and stabilizes at 20 to 25% in adolescence. In old age the percentage of sleep time devoted to REM declines to about 20%. Older adults also show a sharp decline in Stage 3 and Stage 4 NREM sleep.
Experience suggests that sleep has some sort of restorative function. Humans feel refreshed and energized after a good night's sleep, and feel tired and ineffectual when they don't sleep well. But science has had difficulty going beyond this common-sense understanding of sleep. The physiological purpose of sleep continues to be something of a mystery.
The most common way to look for the purpose of sleep is to study people who have been deprived of sleep and measure the degree of impairment in their functioning. A large number of such studies have been done, with surprisingly slim results. Lack of sleep seems to have very little impact on functions such as motor coordination, sensory perception, or reflex activity, and most cognitive functions seem relatively unaffected as well. The biggest impact seems to be on short-term memory and sustained attention, both of which are impaired somewhat by sleep deprivation. This impairment may be due to the subjects' marked tendency to fall asleep for short periods as sleep deprivation is increased. By far, the most common outcome of sleep deprivation is increased sleepiness. As deprivation increases, the pressure to fall asleep intensifies to the point where it is almost impossible to keep subjects awake unless they are monitored constantly.
Role in human health
The quality and quantity of sleep are important indicators of overall health. Sleep is often affected by physical or emotional stress, and sleep disturbances are good indicators that something is amiss with a person. Although the majority of sleep complaints can be traced to psychosocial stress, sleep disturbance can be an important feature of many serious physical or psychological problems as well.
Sleep disturbance occurs in a wide variety of medical problems, including endocrine disturbances, gastrointestinal disorders, and hypertension. Chronic pain disorders such as arthritis and fibromyalgia also produce sleep disturbances, and sleep disruption is a common feature of a number of neurological disorders. Complaints about sleep are also very common with psychiatric illnesses, especially anxiety disorders and mood disorders, and they also occur in some forms of psychosis. Sleep disruption can be an important indicator of substance abuse. The most obvious case is the abuse of stimulants, such as caffeine or amphetamines, but alcohol abuse can also interfere with sleep, as can the abuse of sedatives.
Inadequate sleep is also a public health issue in its own right. A recent poll indicated that 63% of American adults fail to get the recommended amount of sleep at night, and 69% report frequent sleep problems. Chronic lack of sleep causes daytime sleepiness, which increases the risk of accidents of all types, especially automobile accidents. One estimate suggests that driver sleepiness plays a role in 10% of serious automobile accidents. Lack of sleep also impairs work performance and may contribute to industrial accidents.
Common diseases and disorders
Sleep disorders can be classified as primary or secondary, depending on the presumed cause of the disorder. Primary sleep disorders are those that arise in the absence of other medical or psychiatric conditions, while secondary sleep disorders are likely caused by some other condition.
Some of the more common primary sleep disturbances include the following:
- Primary insomnia: This disorder is defined as difficulty getting to sleep or staying asleep that lasts for over one month. Primary insomnia is often triggered by psychological stress, but it may persist long after a stressful event occurs. It is often related to anxiety about sleep, as well as poor sleep hygiene.
- Narcolepsy: Narcolepsy is characterized by periodic attacks of uncontrollable sleepiness, sometimes triggered by strong emotions. Patients with narcolepsy often experience cataplexy, a sudden loss of muscle tone, which can result in falling and injuries. Other symptoms of narcolepsy include hallucinations and sleep paralysis. Narcolepsy occurs in around.04% of the general population.
- Breathing-related sleep disorders: This is a group of disorders that are all characterized by disturbed sleep due to periodic disruptions in breathing. The most common form is obstructive sleep apnea (OSA) syndrome, in which sleep is marked by periodic blockage of the upper airway. This disorder may affect 2 to 4% of the general population.
- Nocturnal myoclonus and restless leg syndrome: These are characterized by night-time discomfort and movement of the lower extremities. In nocturnal myoclonus, the person may be awakened by twitching or cramps in the legs. In restless leg syndrome, patients usually report a "crawly" feeling and the urge to move their legs. Both disorders interfere with sleep, and patients may complain of insomnia or daytime sleepiness.
- Circadian rhythm sleep disorders: In these disorders the timing of sleep is disturbed, so that the person's sleep schedule does not fit with external social demands. Shift work and long-distance travel can contribute to
- these disorders, but they are also common in elderly people. Often these disorders are treated by light exposure and other efforts to "reset" the patient's internal clock.
- Sleep terror disorder: This is a disorder in which the patient wakes up physically aroused and screaming or crying. Although these episodes resemble nightmares, they usually occur during NREM sleep rather than during the normal dreaming of REM sleep. The patient often cannot recall the episode the next morning. Sleepwalking disorder is a similar condition, involving complex movements and activities during sleep. It also occurs during NREM sleep. Both disorders are more common in children than adults.
Cataplexy—An abrupt and reversible loss of muscle tone. Cataplexy is one of the key symptoms of narcolepsy.
Circadian rhythm—A body rhythm that occurs in a 24-hour cycle. Sleep, body temperature, and some endocrine functions exhibit circadian rhythms.
Electroencephalogram (EEG)—A recording of electrical activity in the brain.
Insomnia—Difficulty falling asleep or staying asleep.
Nocturnal myoclonus—A sleep disorder in which sleep is disturbed by twitching or cramps in the leg muscles. Nocturnal myoclonus is also known as periodic limb movements in sleep (PMLS).
Non-rapid eye movement (NREM) sleep—Stages of sleep during which rapid eye movements do not occur. The majority of sleep consists of the four stages of NREM sleep.
Rapid eye movement (REM) sleep—A stage of sleep during which the sleeper's eyes move back and forth rapidly. Most dreams occur during REM sleep.
Restless leg syndrome (RLS)—A sleep disorder in which the person is awakened by uncomfortable "crawly" feelings in the legs.
Sleep apnea—Temporary cessation of breathing during sleep.
Sleep hygiene—A set of behaviors associated with the timing and conditions of sleep. Good sleep hygiene involves setting a regular sleep schedule, avoiding bedtime stress, and restricting activities so that the bed becomes a place for sleeping.
Sleep paralysis—A sudden inability to move that occurs at the point of falling asleep or awakening.
Lashley, Felissa R., and M. de Menses. "Sleep Enhancement." In Nursing Interventions: Effective Nursing Treatments, ed. G.M. Bulechek and J.C. McCloskey. Philadelphia:W.B. Saunders, 1999.
Poceta, J. Steven, and Merrill M. Mitler, eds. Sleep Disorders: Diagnosis and Treatment. Totowa, NJ: Humana Press, 1998.
Shneerson, John M. Handbook of Sleep Medicine. Oxford: Blackwell Science, 2000.
Borbely, Alexander A., and Giulio Tononi, "The quest for the essence of sleep." Daedalus 127 (Spring 1998): 167.
Martin, Jennifer, Tamar Shochat, and Sonia Ancoli-Israel, "Assessment and treatment of sleep disturbances in older adults." Clinical Psychology Review 20 (August 2000): 783-805.
Myslinski, Norbert R., "In the Arms of Morpheus." World and I 15 (December 2000): 130.
National Sleep Foundation. 1522 K Street NW. Suite 500, Washington, DC 20005. <http://www.sleepfoundation.org>.
"Less Fun, Less Sleep, More Work: An American Portrait." 2001 Sleep in America Poll results. National Sleep Foundation. March 2001. 27 March 2001. <http://www.sleepfoundation.org/PressArchives/lessfun_lesssleep.html.>.
Denise L. Schmutte, Ph.D.