Heart Rate During Sleep: Nocturnal Patterns and Autonomic Shifts

Heart rate does not monotonically slow during sleep. It varies dynamically across sleep stages due to autonomic shifts:

• NREM Stage N1/N2: Parasympathetic dominance → HR declines below waking baseline (10–20 bpm drop).
• NREM Stage N3 (slow-wave): Maximal vagal tone; lowest HR of night (often 40–60 bpm in healthy adults).
• REM sleep: Sympathetic surges (phasic) → HR increases, becomes variable, can exceed waking rates. Associated with dreaming, autonomic instability.

Clinical Relevance

• Normal nocturnal dipping (HR & BP) predicts cardiovascular health.
• Loss of dipping / REM tachycardia associated with OSA, heart failure, autonomic neuropathy.
• Wearable HR tracking shows population-level U-shaped curve across the night.

Measurement

Polysomnography (ECG) is gold standard; validated wearables (PPG) correlate well for trends.

Sleep is an active autonomic state, not simply "rest."