Sleep

Sleep is a behavioral state that is a natural part of every individual’s life. We spend about one-third of our lives asleep. Nonetheless, people generally know little about the importance of this essential activity. Sleep is not just something to fill time when a person is inactive. Sleep is a required activity, not an option. Even though the precise functions of sleep remain a mystery, sleep is important for normal motor and cognitive function. We all recognize and feel the need to sleep. After sleeping, we recognize changes that have occurred, as we feel rested and more alert.

Sleep is not a passive event, but rather an active process involving characteristic physiological changes in the organs of the body. Scientists study sleep by measuring the electrical changes in the brain using electroencephalograms (EEGs).

Typically, electrodes are placed on the scalp in a symmetrical pattern. The electrodes measure very small voltages that scientists think are caused by synchronized activity in very large numbers of synapses (nerve connections) in the brain’s outer layers (cerebral cortex). EEG data are represented by curves that are classified according to their frequencies. The wavy lines of the EEG are called brain waves.

An electrooculogram (EOG) uses electrodes on the skin near the eye to measure changes in voltage as the eye rotates in its socket. Scientists also measure the electrical activity associated with active muscles by using electromyograms (EMGs).

In this technique, electrodes are placed on the skin overlaying a muscle. In humans, the electrodes are placed under the chin because muscles in this area demonstrate very dramatic changes during the various stages of sleep.

In practice, EEGs, EOGs, and EMGs are recorded simultaneously on continuously moving chart paper or digitized by a computer and displayed on a high-resolution monitor. This allows the relationships among the three measurements to be seen immediately. The patterns of activity in these three systems provide the basis for classifying the different types of sleep.

States of Sleep

Studying these events has led to the identification of two basic stages, or states, of sleep: non rapid eye movement (NREM) and rapid eye movement (REM).

Sleep is a highly organized sequence of events that follows a regular, cyclic program each night. Thus, the EEG, EMG, and EOG patterns change in predictable ways several times during a single sleep period. NREM sleep is divided into four stages according to the amplitude and frequency of brain wave activity. In general, the EEG pattern of NREM sleep is slower, often more regular, and usually of higher voltage than that of wakefulness. As sleep gets deeper, the brain waves get slower and have greater amplitude. NREM Stage 1 is very light sleep; NREM Stage 2 has special brain waves called sleep spindles and K complexes; NREM Stages 3 and 4 show increasingly more high-voltage slow waves. In NREM Stage 4, it is extremely hard to be awakened by external stimuli. The muscle activity of NREM sleep is low, but the muscles retain their ability to function. Eye movements normally do not occur during NREM sleep, except for very slow eye movements, usually at the beginning. The body’s general physiology during these stages is fairly similar to the wake state.

The EEG recorded during REM sleep shows very fast and desynchronized activity that is more random than that recorded during NREM sleep. It actually looks similar to the EEG (low voltage with a faster mix of frequencies) from when we are awake. REM sleep is characterized by bursts of rapid eye movements. The eyes are not constantly moving, but they dart back and forth or up and down. They also stop for a while and then jerk back and forth again. Always, and just like waking eye movements, both eyes move together in the same direction. Some scientists believe that the eye movements of REM sleep relate to the visual images of dreams, but why they exist and what function they serve, if any, remain unknown. Additionally, while muscle tone is normal in NREM sleep, we are almost completely paralyzed in REM sleep. Although the muscles that move our bodies go limp, other important muscles continue to function in REM sleep. These include the heart, diaphragm, eye muscles, and smooth muscles such as those of the intestines and blood vessels. The paralysis of muscles in the arms and legs and under the chin show electrical silence in REM sleep. On an EMG, the recording produces a flat line. Small twitches can break through this paralysis and look like tiny blips on the flat line.

Sleep is a cyclical process. During sleep, people experience repeated cycles of NREM and REM sleep, beginning with an NREM phase. This cycle lasts approximately 90 to 110 minutes and is repeated four to six times per night. As the night progresses, however, the amount of deep NREM sleep decreases and the amount of REM sleep increases.

Sleep Disorders

Problems with sleep can be due to lifestyle choices and can result in problem sleepiness—that is, feeling sleepy at inappropriate times. Environmental noise, temperature changes, changes in sleeping surroundings, and other factors may affect our ability to get sufficient restful sleep. Short-term problem sleepiness may be corrected by getting additional sleep to overcome the sleep deficit. In other cases, problem sleepiness may indicate a sleep disorder requiring medical intervention. Alcohol abuse can cause or exacerbate sleep disorders by disrupting the sequence and duration of sleep states. Alcohol does not promote good sleep, and consuming alcohol in the evening can also exacerbate sleep apnea problems.

More than 70 sleep disorders have been described, the most common of which are:

• Insomnia, the most prevalent sleep disorder, is characterized by an inability to fall asleep and/or by waking up during the night and having difficulty going back to sleep. Primary insomnia is more common in women than men and tends to increase with age. Short-term or transient insomnia may be caused by emotional or physical discomfort, stress, environmental noise, extreme temperatures, or jet lag, or may be the side effect of medication. Secondary insomnia may result from a combination of physical or mental disorders, undiagnosed or uncontrolled sleep disorders (that is, sleep apnea, restless legs syndrome, narcolepsy, and circadian rhythm disorders), and effects of prescription or nonprescription medications. Treatment will differ for primary and secondary causes of insomnia. Treatment may include behavioral aspects, such as following a specific nighttime routine, improving sleep environment, reducing caffeine and alcohol intake, or reducing afternoon napping. Pharmacological treatments may alleviate symptoms in specific cases. Some individuals try to overcome the problem of insomnia by drinking alcoholic beverages. Alcohol inhibits REM sleep and the deeper, restorative stages of sleep, and therefore does not promote good, restful sleep.

• Obstructive sleep apnea (OSA) is a potentially life-threatening disorder in which breathing is interrupted during sleep. An estimated 12 million Americans have OSA. This condition may be associated with bony or soft tissue that limits airway dimensions and is made worse in the presence of excess fatty tissue. Repetitive episodes of no effective breath, very shallow breaths, or adequate breaths but with high airway resistance can occur 20 to 30 times per hour or more. These episodes cause temporary drops in blood oxygen and increases in carbon dioxide levels, which lead to frequent partial arousals from sleep. Limitations in upper-airway dimensions are typically associated with chronic loud snoring. The frequent arousals result in ineffective sleep and account for the chronic sleep deprivation and the resultant excessive daytime sleepiness that is a major hallmark of this condition. Additional effects include morning headaches, high blood pressure, heart attacks, heart-rhythm disorders, stroke, and decreased life expectancy. OSA also occurs in children and is generally related to enlarged tonsils or adenoids. It occurs equally often in boys and girls and is most common in preschool-age children. Because many of the factors contributing to OSA appear to have significant genetic influences (such as bony dimensions of upper airways), genetic risk factors are likely important in the occurrence of OSA. Treatment for adult OSA can include behavioral therapy (losing weight, changing sleeping positions, and avoiding alcohol, tobacco, and sleeping pills), use of mechanical devices (continuous positive airway pressure to force air through the nasal passages, or dental appliances that reposition the lower jaw and tongue), and surgery to increase the size of the airway.

• Restless legs syndrome (RLS) is a neurologic movement disorder that is often associated with a sleep complaint. People with RLS have unpleasant leg sensations and an almost irresistible urge to move the legs. Symptoms are worse during inactivity and often interfere with sleep. RLS sufferers report experiencing creeping, crawling, pulling, or tingling sensations in the legs (or sometimes the arms) that are relieved by moving or rubbing them. Sitting still for long periods becomes difficult; symptoms are usually worse in the evening and night and less severe in the morning. Periodic leg movements, which often coexist with restless legs syndrome, are characterized by repetitive, stereotyped limb movements during sleep. Periodic limb movement disorder can be detected by monitoring patients during sleep. Some people with mild cases of RLS can be treated by exercise, leg massages, and eliminating alcohol and caffeine from the diet. Others require pharmacological treatment, and it may take some time to determine the right medication or combination of medications for the individual. Estimates suggest that RLS may affect between 10 and 15 percent of the population.

• Narcolepsy is a chronic sleep disorder that usually becomes evident during adolescence or young adulthood and can affect both men and women.  In the United States, it affects as many as 250,000 people, although fewer than half are diagnosed. The main characteristic of narcolepsy is excessive and overwhelming daytime sleepiness (even after adequate nighttime sleep). A person with narcolepsy is likely to become drowsy or to fall asleep at inappropriate times and places. Daytime sleep attacks may occur with or without warning and may be irresistible. In addition, nighttime sleep may also be fragmented. Three other classic symptoms, which may not occur in all people with narcolepsy, are cataplexy (sudden muscle weakness often triggered by emotions such as anger, surprise, laughter, and exhilaration), sleep paralysis (temporary inability to talk or move when falling asleep or waking up), and hypnagogic hallucinations (dreamlike experiences that occur while dozing or falling asleep). People with narcolepsy have difficulty staying awake, and in extreme conditions, narcoleptic episodes can occur during periods of activity. Narcolepsy is not the same as simply becoming tired or dozing in front of the TV after a day’s work.

REM sleep in people with narcolepsy frequently occurs at sleep onset instead of after a period of NREM sleep. Consequently, researchers believe that the symptoms of narcolepsy result from a malfunction in some aspect of REM sleep initiation. Some scientists believe that the immune system causes narcolepsy by attacking the nervous system (that is, an autoimmune response). In this view, exposure to an unknown environmental factor results in an immune response against nerve cells in the brain circuits that control arousal and muscle tone. The discovery of a narcolepsy gene in dogs indicates that genetic risk factors for narcolepsy may also be pertinent in humans. Studies of narcoleptic dogs suggest that altered receptors for a specific neurotransmitter in the hypothalamus can cause cataplexy and the other symptoms of narcolepsy. Many individuals with narcolepsy appear to have a deficiency of this hypothalamic transmitter. There is no definitive cure for narcolepsy, but several treatment options alleviate various symptoms. Treatment is individualized depending on the severity of the symptoms, and it may take weeks or months for the optimal regimen to be worked out. Treatment is primarily by medications, but lifestyle changes are also important.

• Parasomnias are sleep disorders that involve a range of behaviors that occur during sleep. These include sleepwalking, sleep talking, enuresis (bed-wetting), and sleep terrors, which are NREM disorders that occur early in the night. Many of the parasomnias (including sleepwalking, sleep talking, and sleep terrors) are more common in children. Children generally have no memory of such events, usually do not require treatment, and usually outgrow the disorder. Enuresis may respond to drug treatment, and like other parasomnias in children, generally resolves as the child becomes older.

REM sleep behavior disorder is a parasomnia that occurs later in the night than NREM disorders. It differs from the parasomnias discussed previously because it usually affects middle-aged or elderly individuals. Frequently, sufferers will also have a neurological disorder. The temporary muscle paralysis that normally occurs during REM sleep does not occur in this disorder. Because the muscles are not paralyzed, individuals may act out potentially violent behaviors during sleep and cause injuries to themselves or their bed partners.

Test your knowledge