The link between how people sleep and how long they live is large and repeatedly found, but almost none of it has been tested by assigning people to sleep more. Firmest evidence: alertness, clear thinking, mood, and treating broken sleep. For someone already doing the rest well, the target is the same hours at the same time every night. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Sleep Regularity Index (SRI) | 85 or higher on a 0–100 scale | Timing consistency predicts mortality more strongly than duration does |
| Average nightly sleep duration | 7.0–8.5 hours of actual sleep | The exposure the entire epidemiology is built on |
| Sleep efficiency | 85–92% | Separates short sleep from fragmented sleep, and flags over-extension |
| Insomnia Severity Index (ISI) | 0–7 | Validated symptom scale that tracks response to behavioural treatment |
| Epworth Sleepiness Scale (ESS) | 0–8 | Detects residual daytime sleepiness that adequate opportunity has not resolved |
| STOP-Bang score | 0–2 | Screens for obstructive sleep apnea before any self-directed protocol |
| Fasting glucose | 75–86 mg/dL | Sleep restriction lowers insulin sensitivity within days |
| HbA1c | 4.9–5.4% | Integrates months of glucose exposure, so it survives single-night noise |
| hs-CRP | Below 1.0 mg/L | Tracks the inflammatory signal associated with disturbed sleep |
| Morning blood pressure | Below 120/80 mmHg | Short sleep tracks with incident hypertension in cohort data |
| Overnight heart-rate variability (HRV) | No established population target; stability or rise against the individual's own 30-day baseline | Early, sensitive signal of alcohol, late meals, overtraining or illness |
| Morning resting heart rate | Within 3 beats per minute of the individual's own baseline | Rises with alcohol, late training and illness before subjective symptoms appear |
| Serum ferritin | 50–100 ng/mL in women, 50–150 ng/mL in men | Low iron stores drive restless legs, a common and treatable cause of fragmented sleep |
| Total testosterone (men) | 500–800 ng/dL | Sleep restriction measurably lowers it, making it a concrete response marker |
| Serum TSH | 0.5–2.0 mIU/L | Thyroid dysfunction mimics both insomnia and excessive sleepiness |
Cadence: Baseline over at least fourteen nights before any change. Sleep diary and device trends weekly; symptom scales at four and twelve weeks; blood panel and blood pressure at three months, then every six to twelve months.