Skipping 90 Minutes Of Sleep A Night Made People Gain Weight, Not Fat

Short Sleep Quietly Added Pounds That Fat Scans Couldn’t Find

There are plenty of reasons one might find themselves up a bit later than usual. A late-night scroll session, an extra hour at the desk, or perhaps a TV show that runs long, just to name a few examples. After all, shaving 90 minutes off a night’s sleep barely registers as sacrifice for most working adults.

Now, however, new research published in the journal Annals of Internal Medicine found that losing out on sleep, repeated night after night for six weeks, was enough to nudge the body toward extra pounds, even without a detected change in moderate or vigorous activity. The study did not directly track how much participants ate. Stranger still, researchers did not detect a matching rise in body fat percentage, pointing to something less straightforward than simple overeating.

Researchers at Columbia University Irving Medical Center tracked 95 adults through a controlled experiment comparing six weeks of shortened sleep against six weeks of adequate sleep. Every participant already carried some risk for heart or metabolic disease and typically slept seven or more hours nightly before joining. Instead of asking people to recall sleep habits afterward, scientists monitored them in real time with wrist sensors and whole-body scans.

Body weight, waist size, and a hormone that helps regulate appetite and reflects the body’s energy stores all rose during the sleep-restricted stretch compared with the well-rested one, while time spent sitting also increased. Together, these findings provide randomized evidence that ordinary, mild sleep loss can raise body weight even when scans show no detectable change in the proportion of the body made up of fat.

Six Weeks of Mild Sleep Loss Cut Nightly Rest by Nearly 80 Minutes

Scientists combined data from two clinical trials, both using a crossover design, meaning every participant experienced both sleep conditions rather than being split into separate groups. This let each person serve as their own comparison, limiting the influence of genetics or lifestyle differences.

Participants spent six weeks maintaining their usual, adequate sleep schedule and, at a separate point, six weeks delaying their bedtime by about 90 minutes per night while keeping the same wake time, with a multiweek break separating the two phases. Wrist actigraphy devices, similar to fitness trackers, recorded sleep and activity around the clock, while sleep diaries filled in details the devices couldn’t capture.

On average, participants lost 78 minutes of sleep per night during the restriction phase, close to the 90-minute target. Whole-body MRI scans, considered a gold standard for measuring fat and muscle, were taken before and after each six-week period, and blood draws captured hormones tied to hunger and fullness, including leptin, ghrelin, and GLP-1. Roughly three-quarters of participants were women, and researchers also tracked whether effects showed up differently by gender and menopausal status.

Sleep Loss and Weight Gain Showed Up on the Scale, Not in the Fat Scans

By the end of the sleep-restricted stretch, participants weighed about a pound more, on average, than they did after adequate sleep, and their waistlines expanded by roughly a fifth of an inch. Whole-body volume, measured by MRI, increased as well. Researchers found no detectable change in the share of whole-body volume made up of fat or skeletal muscle, meaning the scale moved without a measurable shift in those proportions, at least within the six-week window studied.

Leptin, a hormone that signals how much energy the body has stored, climbed alongside the weight gain, matching what’s known about how the hormone responds to changes in body mass. Ghrelin, a hunger-signaling hormone, trended lower, though not by a meaningful margin, and GLP-1, tied to fullness, barely budged.

Time spent in moderate or vigorous exercise stayed the same regardless of sleep condition, but sedentary time crept up by more than 17 minutes a day during the short-sleep phase. Among the 34 participants whose daily energy expenditure was measured using a specialized water-tracing method, researchers found no meaningful difference between the two sleep conditions, meaning they weren’t burning extra energy to offset staying awake longer. The findings suggest that increased sitting and possibly greater food intake may have contributed, although the study did not directly measure how much participants ate.

This Kind of Mild Sleep Loss Is Already Common Among American Adults

Roughly 78% of American adults who sleep short of seven hours land around six, meaning the sleep loss tested here reflects an everyday pattern rather than an extreme scenario. Researchers concluded that clinicians should factor sleep duration into weight management conversations and encourage patients to protect adequate sleep throughout adulthood, rather than treating it as expendable.

None of this means one late night wrecks a metabolism, and the amount of weight gained here, about half a kilogram, mirrors typical annual weight creep already seen in young and middle-aged adults. The surprise is that researchers detected weight gain without detecting the body-fat percentage shift they expected, a sign that the study could not fully explain what accounted for the added weight during the six-week period. For anyone treating sleep as the easiest corner to cut when a schedule gets tight, that missing piece is worth paying attention to.

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Disclaimer: This article summarizes findings from a peer-reviewed study for general audiences and is not medical advice. Anyone with questions about sleep, weight, or cardiometabolic health should speak with a qualified health care provider.

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