Sleep, stress & behaviour

The strongest evidence is randomised: giving chronically short sleepers about one extra hour a night cut their intake by ~270 kcal/day (Tasali 2022, measured by the gold-standard doubly-labelled-water method), and they lost weight over two weeks without dieting. Mechanistically, short sleep lowers the satiety hormone leptin, raises hunger hormone ghrelin, and makes junk food more rewarding. Evidence ●●●●○

Dieting on short sleep wrecks the quality of loss. In a controlled trial, the same calorie deficit under 5.5 vs 8.5 hours of sleep cut fat loss by 55% and increased muscle loss — the fat share of weight lost fell from 56% to 25%. Same diet, same scale drop, opposite body composition. Aim for ~7–9 hours.

Cortisol clearly drives central fat only when pathologically high (Cushing’s syndrome — rare, ~13 per million). For the everyday stress most people have, the cortisol–abdominal-fat correlation is small (r ≈ 0.10, ~1% of the variance) and partly runs backwards, since carrying more fat itself raises cortisol turnover. ‘Cortisol belly’ as a mass diagnosis overstates the evidence. Evidence ●●○○○ for everyday stress → fat

Where stress genuinely touches weight is behavioural: it shifts a susceptible subset (~35–40% of people) toward palatable high-fat, high-sugar food — while a similar fraction eat less, so ‘stress makes you fat’ is not universal. Cortisol-blocking weight-loss supplements are unsupported: the marketers of CortiSlim were sued by the US FTC and settled for $12 million.

The single strongest behavioural predictor of weight loss. In a landmark review, all 15 studies of food logging found it linked to more loss, in a clear dose-response — more consistent tracking, more loss. Frequent self-weighing tracks with success too (in a 10,000-user cohort, only daily weighing was associated with loss). The point is awareness and early course-correction, not surveillance. Evidence ●●●●●

New habits take a median ~66 days to feel automatic (range 18–254), and missing one day doesn’t reset the clock — so slow progress is normal, not failure. Structured programs reliably beat going it alone: intensive programs (≥12–16 contacts) roughly double the odds of a ≥5% loss, and longer programs beat shorter ones. For environment design, smaller bowls and serving spoons beat the over-hyped ‘small plate’.