How does poor sleep specifically damage autonomic function?

Sleep is the primary autonomic restoration period. During slow-wave (N3) deep sleep, sympathetic tone falls to its daily minimum, vagal tone reaches its maximum, cortisol secretion is suppressed, and the cardiovascular system undergoes the lowered heart rate and blood pressure that is essential for arterial wall recovery. During REM sleep, autonomic activity is phasic and complex, with episodic sympathetic bursts that are normal in the context of sustained REM organization but damaging if sleep architecture is fragmented and REM intrudes repeatedly into lighter stages.

Chronically poor sleep, fragmented architecture, insufficient N3 and REM duration, or sleep curtailed below six hours, prevents the autonomic restoration window from completing. The result: each day begins with a sympathetic baseline that is incrementally higher than the previous day's. The HRV decline that men associate with overtraining or stress is as often a consequence of sleep quality deterioration as of training load, and the two are frequently confused. A man who reduced his alcohol intake and extended his sleep from six to seven and a half hours of quality sleep recovered more HRV in eight weeks than any exercise protocol produced, in a pattern I have observed frequently in clinical practice. Sleep is not a rest from health. It is where health is built. (Besedovsky et al., Pflugers Archiv, 2012)

Cardiologist's calibrated position, Solid (1) for sleep as the primary autonomic restoration mechanism.

What to do: If your HRV is declining and your sleep is consistently below 6.5 hours or your sleep architecture is fragmented (confirmed by wearable or suspected from morning fatigue), address the sleep before adding any other autonomic protocol.

For the full picture, read Autonomic Sovereignty White Paper.