Tuning daytime and evening light did not improve older adults' sleep
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.
Light alone barely moved young adults' clocks; a combined package did
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.
Eating late does change your metabolism. Whether it makes you fatter is less clear.
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.
Irregular sleep is sticky: after six irregular months, it continued 80% of the time
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.
Night shift work is a 'probable' carcinogen. The human evidence remains far from settled
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.
Blue-light glasses: what 17 randomised trials could and could not show
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.
For jet lag, the evidence on melatonin is unusually strong. Timing is the catch.
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.
Delaying school start times bought 69 minutes of sleep. The mood effect was much smaller.
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.
Elite karate athletes lost the most performance at 5pm in week four of Ramadan
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.
Weekend lie-ins tracked with fewer depressive symptoms in a national youth survey
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.
Twenty years of night shifts, and a 32% higher risk of heart, kidney or metabolic disease
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.