The most restorative stage of sleep is NREM-3.
In a sleep cycle you progress through the stages:
- Initial sleep induction: NREM-1 -> NREM-2 -> NREM-3
- Full sleep cycle: NREM-3 -> NREM-2 -> NREM-1 -> REM -> NREM-1 -> NREM-2 -> NREM-3.
A full sleep cycle takes approximately 90 minutes, but the time you spend in each stage changes during the night. At first you spend more time in stage 3, but as your body rests it will spend more and more time in REM.