Before you optimise, know what you're aiming at. A handful of numbers describe sleep — most trackers report them — but only a few actually matter, and chasing the rest is how people give themselves sleep anxiety.
Aim for 7–9 hours of actual sleep; both too little and habitually too much track with worse health (a U-shaped curve). But timing regularity may matter as much as duration — going to bed and waking at consistent times predicts lower mortality even after accounting for how long you sleep (the Sleep Regularity Index work). Consistency is the most underrated lever in sleep, and the cheapest. Evidence ●●●●○
You cycle through light (N1/N2), deep (N3, slow-wave), and REM roughly every 90 minutes. Deep sleep — physical restoration and glymphatic clearance — is front-loaded early in the night; REM — emotional processing and memory — is back-loaded toward morning. That's why cutting sleep short mostly steals REM, and going to bed too late steals deep. Rough norms: ~13–23% deep, ~20–25% REM. Evidence ●●●●○
Sleep efficiency (time asleep ÷ time in bed; aim >85%), sleep-onset latency (aim 10–20 min — falling asleep the instant your head hits the pillow signals debt, not virtue), and WASO (wake after sleep onset). Plus the recovery signals: HRV and resting heart rate (higher overnight HRV and lower RHR mean better recovery), respiratory rate, overnight SpO2, and body temperature. Read trends over weeks, never single nights. Evidence ●●●○○ as consumer metrics
Sleep debt is real and it accumulates. You can partially repay it with recovery sleep, but weekend catch-up doesn't fully undo weekday restriction, and the performance hit keeps creeping up while your sense of how impaired you are flatlines (the classic chronic sleep-restriction studies). The lesson: it's far cheaper to avoid building the debt than to repay it. Evidence ●●●●○
Fundamentals beat metrics
Enough hours + regular timing + a dark, cool, quiet room outweigh every deep-sleep percentage you'll ever obsess over. If a metric isn't going to change what you do tonight, it's entertainment, not information.
The Sleep Guide