AP Psychology 1.5: Sleep

Consciousness is your awareness of your own thoughts, feelings and behavior and of events around you, in your internal world and the external one. That awareness comes in levels. Reading a hard paragraph, you are highly aware of it. Half asleep on a bus, you might still catch your stop being called while missing everything else. Sleep and wakefulness are two types of consciousness, not consciousness switched on and off.

Moving between the two follows a schedule. The sleep/wake cycle is a circadian rhythm, a biological rhythm that in humans runs on about a 24-hour cycle. It pushes alertness down at some hours and up at others, which is why a person kept awake all night can feel sharper at 11 a.m. than at 4 a.m.

The rhythm keeps its own time, so it can fall out of step with a person's life. Jet lag is one disruption: fly from Los Angeles to Paris and your body runs on Los Angeles time for days, sleepy in the Paris afternoon and wide awake at Paris midnight. Shift work is another. A baker who starts at 3 a.m. or a nurse on night shifts has to be alert when the rhythm is pushing toward sleep, and has to sleep in daylight when it is pushing toward waking.

Sleep researchers record brain waves with an EEG, sensors on the scalp that track the brain's electrical activity. Each stage of sleep has its own EEG pattern, and that pattern is how the stages are told apart.

NREM sleep (non-REM) comes in Stages 1 through 3. As a person drifts into Stage 1, the waves slow from their waking pattern, and hypnagogic sensations can come with it: a feeling of falling, a sudden jerk, or a brief image or sound that is not there. Stage 2 is deeper. In Stage 3, the deepest, the waves are large and slow, and the sleeper is hard to wake.

REM sleep, named for its rapid eye movements, is called paradoxical sleep. Its EEG shows fast waves much like wakefulness, yet the body is at its most relaxed, with the muscles nearly limp. Dreaming typically happens in REM.

A night runs through these stages in repeated cycles, and the cycles change. NREM time, the deep Stage 3 most of all, shrinks from one cycle to the next, while REM comes more often and lasts longer toward morning. Take REM away by waking someone each time it starts, and on the next undisturbed night REM increases, as if the body were catching up. That is REM rebound.

Two current theories explain why sleep happens at all. Restoration theory says sleep restores resources the body and brain used up during the day, so a day of hard physical or mental work leaves a person needing more of it. Memory consolidation theory says sleep is when the brain organizes and consolidates memories, making what was learned that day more lasting. On this view, material learned shortly before sleep should be remembered better than the same material followed by the same number of hours awake.

Dreams have theories of their own. Activation-synthesis theory says that during REM the brain stem sends out bursts of neural activity, and the cortex tries to make sense of them by building a story. That would explain why dreams are so often strange, jumping from one scene to another: the story is stitched together on the spot from activity that had no plan.

Consolidation theory of dreams says dreaming is part of the brain's work of sorting and storing memories. That would explain why pieces of recent experience, such as a play a student rehearsed all week or a route a new driver practiced, so often turn up in dreams.

The names overlap, so keep the questions apart: one pair of theories asks why we sleep, the other asks why we dream.

Many disorders interrupt healthy sleep, and their effects on waking life vary. Short or broken sleep can slow reactions and weaken attention and memory the next day, and over time it can wear on physical health. These five are the ones psychologists most often study:

• Insomnia: ongoing trouble falling asleep or staying asleep.
• Narcolepsy: sudden, uncontrollable attacks of sleep during the day, sometimes dropping straight into REM.
• Sleep apnea: breathing stops again and again during sleep, briefly waking the person each time, often without their knowing it. Loud snoring is common.
• Somnambulism, or sleepwalking: walking or doing other things while still asleep, usually in deep NREM sleep early in the night.
• REM sleep behavior disorder: the muscles fail to stay relaxed during REM, so the person acts out dreams, sometimes by kicking or punching.

Treating a sleep disorder can improve how well a person performs while awake and their overall well-being. So can keeping a regular schedule for sleeping and waking.

To tell the five apart, ask when the trouble shows up (getting to sleep, during the night, or in the daytime) and what the body is doing.