Sleep may be restful, but it's a busy time for the brain

Sleep was once thought of as downtime for the brain -- a passive, dormant part of life. But scientists have learned that, in fact, the brain is quite active during sleep.

According to the U.S. National Institute of Neurological Disorders and Stroke, nerve-signaling chemicals called neurotransmitters control whether a person is asleep or awake by acting on different groups of nerve cells, or neurons, in the brain. Neurons in the brainstem, which connects the brain with the spinal cord, produce neurotransmitters such as serotonin and norepinephrine that keep some parts of the brain active while a person is awake.

But other neurons at the base of the brain begin signaling when someone falls asleep. These neurons appear to "switch off" the signals that keep the person awake.

Research also suggests that a chemical called adenosine builds up in the blood while people are awake and causes drowsiness. This chemical gradually breaks down while they're asleep.

During sleep, people usually pass through five phases -- known as stages 1, 2, 3, 4 and REM (rapid eye movement) sleep. The stages progress in a cycle from stage 1 to REM sleep, and then the cycle starts over again with stage 1.

For adults, about 50 percent of total sleep time is spent in stage 2, about 20 percent in REM sleep and the remaining 30 percent in the other stages. Infants, by contrast, spend about half of their sleep time in REM sleep.

Different things happen during the stages:

• Stage 1: This is light sleep, when people drift in and out of sleep and can be awakened easily. The eyes move very slowly, and muscle activity slows. People awakened from stage 1 sleep often remember fragmented visual images. Many also experience sudden muscle contractions, often preceded by a sensation of starting to fall.

• Stage 2: In this stage, eye movements stop and brain waves become slower, with occasional bursts of rapid waves, called sleep spindles.

• Stage 3: At this point, extremely slow brain waves, called delta waves, begin to appear, interspersed with smaller, faster waves.

• Stage 4: By now, the brain is producing delta waves almost exclusively. It is very difficult to wake someone during stages 3 and 4, which together are called deep sleep. There is no eye movement or muscle activity. People awakened during deep sleep do not adjust immediately and often feel groggy and disoriented for several minutes. Some children experience bed-wetting, night terrors, or sleepwalking during deep sleep.

• REM sleep: When we switch into this final stage, breathing becomes more rapid, irregular and shallow, the eyes jerk rapidly in various directions and limb muscles become temporarily paralyzed. Heart rate increases and blood pressure rises. People awakened during REM sleep often describe bizarre and illogical tales: dreams.

The first REM sleep period usually occurs 70 to 90 minutes after someone falls asleep, and a complete sleep cycle takes 90 to 110 minutes, on average. The first sleep cycle each night contains relatively short REM periods and long periods of deep sleep. But as the night progresses, REM sleep periods increase in length and deep sleep decreases. By morning, people spend nearly all their sleep time in stages 1, 2 and REM.

People awakened after sleeping more than a few minutes are usually unable to recall the last few minutes before they fell asleep. This sleep-related form of amnesia is the reason people often forget telephone calls or conversations they had in the middle of the night. And it also explains why people often don't remember their alarms ringing in the morning if they go right back to sleep after turning them off.