Typical late-evening routines that end with sleep usually begin with a tired feeling and a habitual unwinding of daily activities and emotions. Assuming the presence of a comfortable bed and environment, most people fall asleep within 20 minutes.
The initial sleep phase sequence is as follows.
- a relatively short period of non-REM light sleep
- a relatively long period of non-REM deep sleep
- a relatively short period (again) of non-REM light sleep
- a very short period of REM sleep
Then the sequence restarts, sometimes after a very short transition to wakefulness. The sequence repeats 4 to 5 times during the night before the final morning wakeup. A single sequence, known as a sleep cycle, may last from 90 to 120 minutes.