There are two light sleep phases.
Cycle 1: In the first 7 minutes, you are in a phase of light sleep. Your eyes try to stay open and your muscles start to relax.
Cycle 2: You then enter the second phase of light sleep, in which your body becomes relaxed and gets ready to dream.
There are also two light deep phases.
Cycle 3 (slow wave sleep): this phase lasts between 20 and 40 minutes, and allows the body to relax.
Cycle 4 (paradoxical sleep): this is the phase when we dream. It enhances the brain's functions and allows for the creation of memories.
Restorative sleep is characterised by an increase in the length of paradoxical sleep as the night progresses. It gradually replaces the cycle 3.