As you sleep, your brain cycles through four stages of sleep.

– Stages 1 to 3 are what's considered non-rapid eye movement (NREM) sleep, also known as quiet sleep.

– Stage 4 is rapid eye movement (REM) sleep, also known as active sleep or paradoxical sleep.

Each has a unique function and role in maintaining your brain's overall cognitive performance

NREM Stage 1 The first stage of the sleep cycle is a transition period between wakefulness and sleep.

NREM Stage 2 According to the American Sleep Foundation, people spend approximately 50% of their total sleep time during NREM stage 2, which lasts for about 20 minutes per cycle.

NREM Stage 3 Deep, slow brain waves known as delta waves begin to emerge during NREM stage 3 sleep—a stage that is also referred to as delta sleep.

REM Sleep While your brain is aroused with mental activities during REM sleep, the fourth sleep stage, your voluntary muscles become immobilized.