Stages of sleep

Sleep is divided into two main phases:  

Non-REM phase: This phase has no rapid eye movements (REM). This phase itself is divided into 3 states: N1 (drowsiness, 10% of the time), N2 (light sleep, 45% of the time), and N3 (deep sleep, 20% of the time).  

REM phase: This phase has rapid eye movements (REM). This is deep sleep phase in which the brain is very active. The brain stem blocks the motor neurons so the person cannot move. In this phase you dream. REM sleep is considered to be involved in the process of memory and learning.  

In a young healthy adult, non-REM sleep occupies approximately 75% of sleep time and REM the remaining 25%.