What is the single most important sleep metric I should be tracking?

For men over 40, the single most clinically informative sleep metric is deep sleep percentage (N3 as a proportion of total sleep time), because it declines with age, is suppressed by the most common reversible causes (sleep apnea, alcohol, elevated cortisol), and directly governs the hormonal and cardiovascular restoration functions of sleep, targeting above 15% of total sleep as N3 is the most specific benchmark (Mander et al., Neuron, 2017).

Total sleep time tells you very little about restoration. Sleep latency (how long to fall asleep) is useful but not the most important metric. REM percentage is meaningful but relatively preserved in middle age. Deep sleep percentage is where the clinical signal concentrates for men 40–55. Below 10% consistently? Likely OSA, alcohol, or elevated cortisol, all modifiable. Between 10–15%? Suboptimal but addressable with hygiene. Above 15%? You are preserving the restorative function that declines with age.

Honesty Scale: Solid (1) for deep sleep's physiological primacy in men over 40 based on the converging GH, cardiovascular, and metabolic literature.

What to do: Set up your Oura or Garmin to show deep sleep percentage as a primary dashboard metric. Monitor it weekly, not nightly. If it has been below 10% for a month, treat that as a clinical finding, not a data point, and schedule a medical evaluation.

For the full picture, read The Sleep Architecture Deep Dive