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-restorative sleep increases the risk of:

• daytime sleepiness;
• accidents;
• cardiovascular disease;
• diabetes;
• obesity;
• psychiatric symptoms;
• impaired attention and memory.

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2. Definition

Sleep is a reversible physiological state characterized by:

• reduced responsiveness to the environment;
• altered consciousness;
• characteristic electroencephalography (EEG) patterns;
• cyclic alternation between NREM and REM sleep;
• changes in eye movement, muscle tone, autonomic function and metabolism.

Sleep differs from coma because the sleeping person can be awakened.

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3. Regulation of sleep

Normal sleep is controlled by three main timing systems.