During the night, our sleep state alternates between rapid eye movement (REM) and other states (collectively referred to as non-REM).
Non-REM sleep is divided into three stages. The first two stages account for light sleep, the third is deep sleep.
For the typical person, it takes about 45 minutes to slip through the first two stages into the last. In these stages most physiological processes are greatly reduced. It’s very difficult to wake a person in deep sleep. Incidentally, nightmares can occur during this time.
After about 90 minutes, we enter into REM sleep in which brain activity changes dramatically. Our muscles become inhibited but our eyes start flickering, hence Rapid Eye Movement.
In fact, brain activity is said to be more intense in REM than when we are awake. Most dreaming also occurs in REM. The muscle paralysis keeps us from acting out our dreams, even though our eyes move according to what we see in our dreams.
A typical adult will enter REM sleep roughly every 90 minutes. Each time REM lasts a little longer. Upon completion of REM the body starts the sleep cycle again. This nomally repeats 4-5 times per night.
Non-REM is believed to help the brain’s physiological processes reboot, while REM is believed to help us work through psychological needs.
Recent research indicates that REM sleep is necessary for processing new information, i.e. learning. Early in life we spend about 50% of our sleep in REM. But from about the age of 10 we stabilize at about 25%.
When we are sleep deprived we spend more time in non-REM, verifying our body’s need to “reboot”. This also means less time spent in REM sleep, leading to irritability and emotional problems.