Forty-seven women were all given identical guidance: reduce intake by 500 calories per day for six months. Of these, 26 also agreed to adjust when they ate, keeping all meals within a nine-hour period.
By the end of the trial, the two groups had shed a similar amount of weight. However, only the group with the shorter daily eating window showed improvements in planning and problem-solving.
The study was headed by Sue Shapses, a professor at Rutgers University and Rutgers-RWJ Medical Center. Brandon McGuire supported the work as a doctoral student in her laboratory.
The 12-hour American day
Most Americans spread their eating over more than 12 hours each day. At baseline, the women in this study averaged 12.3 hours.
Human physiology runs on an approximately 24-hour pattern, influenced partly by light and darkness and partly by the timing of food. This pattern is known as the circadian rhythm.
The approach of consuming the day’s food within fewer hours is called time-restricted eating.
Because weight loss can itself sharpen thinking, it can be difficult to determine what is due to the eating window versus the weight change. Only a limited number of trials have attempted to disentangle these effects.
Same calories, different hours
Participants were aged 50 to 79, with a mean age of 61. All were overweight or obese; average body mass index was 33. Seventy-two per cent were white.
Women were randomly allocated to groups. The remaining 21 participants continued with their usual eating schedule.
Those assigned to the shorter-window group chose their own timing, provided their total daily eating time was nine hours or less. Most typically ate between 10 a.m. and 6 p.m.
Across the study, the time-restricted group averaged an 8.2-hour daily eating span, while the comparison group ate over an average of 12.6 hours.
At enrolment, all participants completed a reading-based test used to estimate earlier-life cognitive ability. Average performance fell within the expected range.
Planning improved, reaction time did not
The same battery of cognitive tests was administered at week 0 and again at week 24.
On the spatial planning test, scores rose in the shorter-window group but stayed essentially unchanged in the other group. The between-group difference is unlikely to be explained by chance.
On the memory and learning test, the time-restricted group also made fewer errors. For this measure, however, the gap between groups was not large enough to rule out chance.
Neither multitasking nor reaction time shifted in either group.
Women who reduced their eating window by the greatest number of hours experienced the biggest gains on both tests. This relationship remained after the researchers adjusted for other factors.
“Yes, this was surprising, even though a lot of studies in mice show this effect,” Shapses told Earth.com.
“This study is unique because we control for weight reduction (both groups lost the same amount of weight), and it shows that restricting the eating window, has a significant effect on some cognitive outcomes.”
Both groups lost about 15 pounds
On average, the shorter-window group lost 7.5 kg (16.5 pounds), while the comparison group lost 6.1 kg (13.4 pounds).
Across all 47 women, mean weight loss was about 7 kg (15 pounds). The researchers described the two group reductions as similar.
This equates to roughly 8 per cent of body weight, and no one in the trial shortened their eating window without also losing weight.
“We don’t know if this effect would be observed if it were not combined with moderate (around 8 percent) weight loss,” Shapses said.
Improved sleep is another plausible explanation. Participants completed a standard sleep-quality questionnaire twice, but scores neither changed over the six months nor differed between the groups.
Shapses added that there may be an additional advantage to finishing food intake four hours before bedtime while keeping the day’s eating within eight or nine hours.
Limitations of the study
The trial was not originally designed to answer questions about cognition. Shapses explained it was set up to investigate bone outcomes, and those results have been submitted to a journal.
The team assessed cognitive function-rather than dementia-using standardised tests covering planning, memory, multitasking and reaction time.
“This is a small study and limited to older women, so larger studies are needed to confirm these interesting findings,” Shapses said.
There were no male participants, and nobody had a body mass index below 25.
Future research directions
“Circadian rhythm disruption is especially problematic in older adults and may be worse in those with overweight and obesity,” said Shapses.
She intends to explore how meal timing aligns with-or conflicts with-the body’s light–dark cycle.
Because the brain maintains one clock while organs keep their own, she aims to clarify how mealtimes may influence both.
Her next work will centre on two hormones. One is melatonin, measured either from its onset in dim light or across the whole night. The other is the cortisol rise that occurs after waking.
Funding came from the New Jersey Institute for Food, Nutrition and Health and the U.S. Department of Agriculture’s National Institute of Food and Agriculture.
The results were presented at NUTRITION 2026, the annual meeting of the American Society for Nutrition.
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