09/03/2026
Nearly all weight advice begins with what to eat.
In these trials nobody's diet was assigned at all: only bedtime moved, and within six weeks the scale moved with it.
Zuraikat and colleagues (Ann Intern Med 2026) pooled two randomized crossover trials in 95 adults at raised heart and metabolic risk who habitually slept seven hours or more. Each person lived both conditions in random order with a break between: six weeks keeping their full night, and six weeks with bedtime cut by a target of 90 minutes, which worked out to 78 fewer minutes of sleep a night when measured. Because everyone served as their own comparison, genetics, jobs, seasons and personalities cancel out.
More waking hours mean more opportunity to eat and more time to fill, and short sleep is known to tilt appetite and food choice while draining the drive to move. Both showed up here: the short-sleep weeks added 17 minutes of sitting per day, and leptin, a hormone released by fat tissue, rose, which is what you expect when the body is adding to its energy stores.
After six weeks of short sleep, the same people weighed 0.45 kg more than after six weeks of full sleep, about a pound, with a waist about half a centimetre wider and total body volume up by 0.56 litres, roughly a half-litre bottle. Body fat measured by MRI did not change measurably, which the authors read as six weeks being too short for the composition of the gain to show up on a scan. The mirror-image experiment points the same direction: when habitually short sleepers were coached to extend their nights by about 1.2 hours, they spontaneously ate 270 fewer calories a day within two weeks, with no diet prescribed (Tasali et al., JAMA Intern Med 2022).
A pound in six weeks is a modest effect, and nothing here says the gain compounds at that rate for a year; bodies adapt, and the trials cannot see past week six. The participants were adults at elevated risk who normally sleep well, so the result describes what happens when good sleepers lose an hour and a quarter, not what happens to people who have slept badly for decades. And the pooled design had limited power to say who gains most.
Sleep loss of the ordinary, unremarkable kind, the late show, the early alarm, was enough on its own to push weight, waist and sitting time upward in six weeks, in the same people who held steady when they slept fully. For anyone auditing their diet and training while running chronically short on sleep, this is the variable that moved the scale without a single food rule changing.
Zuraikat et al., Ann Intern Med 2026;179(8):1085 · PMID 42407080
Tasali et al., JAMA Intern Med 2022;182(4):365 · PMID 35129580
