What are the stages of sleep that create our natural sleep architecture?

People used to think that sleep was the time when everything slowed down or stopped. Now we know that a lot is happening as we sleep. And we know that all sleep is not the same. There are two major stages of sleep—non-rapid eye movement (NREM) and rapid eye movement (REM) —with dramatically different characteristics. The natural sleep architecture—the way these stages fit together—is an important part of how sleep restores us.

Non-Rapid Eye Movement Sleep About 80% of adult sleep is NREM sleep. NREM sleep is divided into four stages:

Stage 1—the drowsy transition from waking to sleeping
Stage 2—intermediate sleep, when arousal is more difficult
Stage 3—the beginning of "deep," or slow-wave, sleep
Stage 4—the deepest sleep, when there is little contact with external sensations

During NREM sleep:

Brain activity decreases.
Blood pressure decreases.
Respiration (breathing) decreases.
Heart rate slows.

The time spent in deep sleep—stages 3 and 4—decreases throughout our lifetime. By age 75, stage 4—the deepest sleep—may be completely absent.

Rapid Eye Movement Sleep
REM sleep is often known as the dreaming stage of sleep, although dreams actually occur during all sleep stages. Unlike NREM sleep, REM sleep involves a high level of mental and physical activity, including:

Increased brain activity
Increased and variable blood pressure
Increased and variable heart rate
Increased blood flow to the brain
Increased and variable respiration

In the newborn baby, more than 50% of sleep may be REM sleep. By the age of 2 years, the proportion of REM sleep decreases to 20% to 25% and remains constant throughout adulthood.

Sleep Architecture Sleep usually begins with a cycle that consists of 80 minutes of NREM sleep followed by 10 minutes of REM sleep. This 90-minute cycle is repeated three to six times each night. With each cycle, the amount of slow-wave sleep decreases and the proportion of REM sleep increases. Research indicates that different sleep stages may serve different functions. Although the reasons for the different types of sleep are still being studied, it is clear that we need each type. Getting the right amount of sleep is important, but getting the right kind of sleep is important, too.