Understanding Sleep Stages

Excerpted from
American Academy of Pediatrics Guide to Your Child's Sleep : Birth Through Adolescence
By George J. Cohen, M.D., The American Academy of Pediatrics

Although medical scientists still do not fully understand all of the functions of sleep, its benefits are obvious. After a good night's sleep, we awaken feeling rested, refreshed, and alert. The events we experience during waking hours are integrated into our memory as we sleep. Minor aches and pains often disappear during sleep, which gives the body time to repair some of the minor wear-and-tear damage to muscles and other structures.

The effects of insufficient or disturbed sleep are also quite obvious. From time to time, everyone stays up too late or suffers a sleepless night and then feels groggy and out of sorts the next day. Children who chronically fail to get enough sleep do not learn as well as better-rested youngsters. They also have a higher rate of behavior problems. In many cases, overtired children resort to hyperactivity and difficult behavior as a way of fighting off daytime drowsiness.

For parents of newborns or older children with poor sleep habits, the effects of not getting enough sleep can become a constant source of stress. Indeed, new parents often say that chronic lack of sleep is one of the most trying aspects of adjusting to parenthood. In the following chapters, you'll find practical advice on how to deal with common sleep problems and help you and your entire family get a good night's rest. First, however, let's consider some of the basics about sleep.

To Sleep, Perchance to Dream

Virtually every living creature needs to sleep-at least some of the time. And each creature seems to have its own unique sleep pattern. Many of these patterns have evolved over the eons as a response to environ mental factors. For example, many creatures share our human preference for sleeping mostly at night, when it is dark, quiet, and relatively safe. But night hunters, such as many of the wild cats, sleep mostly during the day and are awake and active at night, when their prey is easier to catch. Still others doze off periodically both day and night.

The amount of time spent sleeping also varies greatly. Domestic cats, for example, can sleep 20 or more hours a day, with many short naps interspersed between longer snoozes. Dolphins, mammals that, like whales, once lived on land but returned to the water, have developed a unique sleep pattern. In contrast to land mammals, these aquatic mammals need to control their breathing voluntarily when surfacing for air. Dolphins sleep with just one side of the brain at a time, periodically surfacing for air and alternating sleep sides until they have fulfilled the days sleep needs. Then there are the birds, whose sleep includes brief flashes of the kind of nerve activity associated with rapid eye movement (REM) sleep in higher animals (see next section). If birds had complete, prolonged REM sleep with its accompanying muscle paralysis, they would topple off their perches. Reptiles, fish, insects, and worms have periods of apparent rest during which sensory stimuli have to be stronger to make them react. However, although these creatures have sleeplike periods, their nervous systems and metabolisms do not change to sleep patterns as defined in higher animals. Humans and most other land animals have different stages of sleep, which range from light and partially awake to a state of unconsciousness so deep that it is often described as "dead to the world."

The Stages of Sleep and Sleep Cycles

There are two basic types of sleep: REM (rapid eye-movement), the "active" sleep when dreams take place; and non-REM, or "quiet" sleep, which is divided into four stages. Each stage is marked by changes in brain waves, muscle activity, eye movements, heart function, and breathing, all of which can be measured by special instruments. Alternating cycles of non-REM and REM sleep make up the sleep stages that occur throughout the night.

Following is a summary of the typical sleep patterns of adults and children over the age of 3 or 4 years.

Stage I is the brief period (up to 5 minutes) of transition from drowsiness to sleep. Brain activity slows and the eyelids close although the eyes continue to move together slowly beneath the closed lids. A person is easily awakened, often with a start, from this stage. Sometimes he may be aware that he is nodding off; at other times, he may think that he is only daydreaming rather than falling asleep.

Stage II is referred to as light sleep and lasts from 10 to 45 minutes. The brain waves begin to change with the appearance of vertex waves and K-complexes-large, slow waves interspersed with bursts of rapid waves called sleep spindles. K-complexes occur in response to environmental stimuli, and can be followed by arousal.

Stage III marks the transition to a deeper state of sleep. There is a further slowing of the brain waves, and breathing and heartbeat become slow and regular. The muscles relax, and the person lies very still, although there may be some involuntary twitching of the legs. Some people begin to sweat; snoring may also occur.

Stage IV flows smoothly from stage III, and marks the deepest state of sleep. The person is not easily awakened from stage III or IV sleep, and if roused, usually rakes a minute or so to become fully awake. Taken together, stages III and IV may last up to 60 minutes. There is then a gradual return to a lighter, stage II sleep.

REM Sleep occurs after one or two complete cycles of stages I through IV sleep. Often referred to as "active sleep," this is the stage during which most dreams occur. The eyes move rapidly under the closed eyelids, breathing and the heartbeat become less regular, and the muscles are more relaxed, although twitching may increase. The first periods of REM sleep of the night usually last for only a few minutes; as the night goes on, however, REM sleep lengthens. This is why many people awaken in the morning while dreaming, and may feel as though the entire night has been spent dreaming.

Sleep Cycles

Normal adult sleep is marked by recurring cycles of stages III and IV sleep (also referred to as slow-wave sleep), or even the lighter stages I and II, followed by varying periods of REM sleep. (These short, recurring cycles are referred to as ultradian rhythms.) Each cycle lasts an average of 90 minutes, although this varies from one person to another, and even from one night to another. For example, a person who has gone for several nights without getting enough sleep is likely to spend more time in REM and stages III and IV sleep than someone who has been sleeping well.