For how we live.
A small crossover trial found 5 mg of melatonin worsened glucose handling and cut early insulin secretion in people carrying the MTNR1B risk variant, but not in non-carriers.
Probably not ideal. Even moderate light during sleep measurably raises night-time heart rate and next-morning insulin resistance, and in one large study women who slept with a light or TV on gained more weight.
The viral countdown says no caffeine 10 hours before bed, no food at 3, no work at 2, no screens at 1. The caffeine and food steps have real evidence behind them; the rest is thinner.
In a driving study, 200 mg of caffeine followed by a nap under 15 minutes cut simulator incidents to 9% of placebo levels, against 34% for caffeine alone. The order is the point.
That heavy, foggy feeling on waking is often sleep inertia — a normal, temporary state, not a sign you slept badly. It bites hardest when you wake from deep sleep or during the biological night.
When you eat may shape how you sleep, but the science is young. The clearest signal is that a heavy meal close to bedtime disrupts sleep, while simply shrinking your eating window neither reliably helps nor harms it.
The best single randomised trial found morning light beat a sham device over eight weeks, but the effect is most reliable when light is added to an antidepressant rather than used alone.
Lying down soon after a meal will not harm most healthy people, but it is not neutral. It reliably worsens acid reflux, and habitually eating late shifts the body's metabolism in a direction tied to weight gain.
Across the trials, the best-supported remedy is melatonin taken near destination bedtime after crossing several time zones. Light exposure is the other real lever; sleeping pills chase the symptom, not the clock.
One week of five-hour nights cut young men's daytime testosterone by 10–15% in a controlled study. Yet CPAP for sleep apnoea leaves the hormone unmoved, pointing back at body weight.
Two meta-analyses find evening exercise leaves sleep largely unchanged or slightly improved. The real exception is vigorous exercise finishing within about an hour of bedtime.
In UK Biobank participants tracked with wrist light sensors, high nighttime light exposure was tied to thickened, weaker heart muscle and 24–33% higher risks of heart attack, stroke and heart failure.
In a 12-person crossover trial, blue-enriched daytime light with reduced evening blue did not cut time spent awake at night, though it nudged a brainwave marker of sleep.
Two small pilot studies in 18-to-25-year-olds found morning bright light on its own shifted circadian timing little, while adding a timed schedule and evening blue-blockers advanced it about 35 minutes.
A tightly controlled crossover trial found late eating increased hunger, cut waking energy expenditure and shifted fat-tissue gene expression. The population data show a 17% higher odds of obesity.
Twenty months of under-mattress sensor data from 95,819 people found that chronic irregularity was common, persistent, and tracked with 46% higher odds of hypertension.
The IARC calls night shift work probably carcinogenic. A 2026 meta-analysis of nurses shows why 'probably' is careful: the breast cancer signal appears only after decades, and shrinks to nothing once bias is corrected.
A Cochrane review found they may not reduce eye strain and could not determine whether they help sleep. A separate meta-analysis found small effects. Neither found the evidence anyone assumes exists.
A Cochrane review found melatonin reduced jet lag in nine of ten trials. Sleep medicine's own guideline rates it a first-tier option for jet lag disorder. Take it at the wrong hour and it can make things worse.
A meta-analysis of six studies found a moderate effect on sleep duration and a small one on depressive symptoms — with high heterogeneity on both.
A twelve-athlete study tested morning, late-afternoon and late-evening sessions across the fasting month — and found the deficit was specific to time of day, not spread evenly across it.
Among 16-to-24-year-olds in NHANES, those who slept longer on weekends had 41% lower odds of daily depressive symptoms. The survey measured both at the same moment, which limits what can be concluded.
A UK Biobank analysis of 96,365 people found a dose-response relationship between lifetime night-shift exposure and cardiovascular-kidney-metabolic disease — amplified in those who also slept short.