The correct answer is 1
Sleep is divided into several phases, each of which has a specific physiological stage. The five stages of sleep form the cycle, lasting about 90 minutes each. In this way, a sleep cycle is repeated four to five times a night.
Basically, we can divide sleep into two phases: REM (Fast Eye Movement) and NREM (Non-Fast Eye Movement). The state of NREM corresponds to 75% of the sleep period, and is divided into four phases.
