Sleep is divided into two broad types
Sleep is divided into two broad types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.
The sleep cycle of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep. The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called slow-wave sleep.
There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.
What Happens During Non-REM Sleep?
There are three phases of non-REM sleep. Each stage can last from 5 to 15 minutes. You go through all three phases before reaching REM sleep.
Stage 1: Your eyes are closed, but it's easy to wake you up. This phase may last for 5 to 10 minutes.
Stage 2: You are in light sleep. Your heart rate slows and your body temperature drops. Your body is getting ready for deep sleep.
Stages 3: This is the deep sleep stage. It's harder to rouse you during this stage, and if someone woke you up, you would feel disoriented for a few minutes.
During the deep stages of NREM sleep, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system.
As you get older, you sleep more lightly and get less deep sleep. Aging is also linked to shorter time spans of sleep, although studies show you still need as much sleep as when you were younger.
What Is REM Sleep?
REM sleep, also known as paradoxical sleep, is the main occasion for dreams (or nightmares), and is associated with fast brain waves, eye movements and loss of muscle tone.
Usually, REM sleep happens 90 minutes after you fall asleep. The first period of REM typically lasts 10 minutes. Each of your later REM stages gets longer, and the final one may last up to an hour. Your heart rate and breathing typically quickens in REM sleep.
Babies can spend up to 50% of their sleep in the REM stage, compared to only about 20% for adults.
