Normal human sleep

1. Big picture

Normal human sleep is an active, organized biological state, not simply “absence of wakefulness.” It alternates cyclically between non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM).

The key exam sentence:

Normal sleep is organized into repeated NREM–REM cycles lasting about 90 minutes, with approximately 4–6 cycles per night. NREM makes up about 70–80% of total sleep, and REM makes up about 20–25%.

Sleep is essential for:

• brain recovery;
• memory consolidation;
• emotional regulation;
• synaptic homeostasis;
• immune and endocrine regulation;
• cardiovascular and metabolic health.

Chronic insufficient or non-restora