The Five stages of sleep

Sleep medicine specialists have discovered that there are a whole lot of things that go on during sleep.

There are five stages of sleep, according to the National Sleep Foundation, four non-REM stages and REM (rapid eye movement) sleep, and sleepers cycle through the stages about every 90 to 100 minutes.

Stage 1 is the transitional stage, when you're between waking and sleep. Your brain waves and muscle activity slow. Sometimes people's bodies jerk just before they fall asleep.

Stage 2 is a light sleep stage. Eye movements cease. Body temperature drops, and heart and brain activity slows. National Sleep Foundation material says there are periods of muscle tone and muscle relaxation, and that occasional brain-wave spikes, called sleep spindles, occur during this stage of sleep.

Stages 3 and 4 are called delta sleep. These are the deep sleep stages, and this is when body restoration and repair occurs. Temperature drops even further during this phase, brain waves are slow and there is decreased muscle tone. People woken during delta sleep are often groggy and disoriented. Night terrors occur during this sleep stage.

Stage 5 is REM sleep, a period of fast brain waves; rapid, shallow breathing; and the rapid eye movements it's named for. Dreaming, believed to be a way of organizing the day's experiences, occurs during REM sleep. Have you ever wanted to scream during a nightmare and been unable to? Muscles become temporarily paralyzed during REM.

These stages of sleep are repeated 4-6 times during the night, but not in exactly the same ratio.

The first REM sleep is short, just seven minutes or so, but REM sleep takes up a larger and larger portion of the cycles as the night goes on, which is why you're often dreaming when your alarm clock goes off. It's not just the amount of sleep, but also the distribution of sleep stages that's important for health. People who don't get adequate delta sleep or REM sleep wake up feeling less than restored.