Harvard Medical School Study Reveals Irregular Sleep Pattern, High-Fat Diet Can Cause Reduced Glucose Tolerance


Harvard Medical School researchers discovered that recurrent circadian disruption, which occurs in individuals with irregular sleep schedules, can cause reduced glucose tolerance when combined with a high-fat “Western-style” diet.

The discovery builds on over a decade of research led by Charles Czeisler ’74, who serves as a professor in HMS’ Division of Sleep Medicine and teaches a widely popular General Education course on the science of sleep. The paper will appear in the May 2022 issue of the medical journal Metabolism.

Previously, Czeisler’s lab had discovered that the combination of minimal sleep and an irregular sleep pattern disrupted glucose metabolism. This follow-up study was “meant to separate these two components to determine whether the impairment was mainly coming from the sleep restriction or from the circadian disruption,” according to Kirsi-Marja Zitting, the first author on the study.

Czeisler explained that studying these two factors separately required performing two different experiments. The first of these was to limit the amount of sleep participants were allowed without disrupting their circadian rhythms, which surprisingly seemed to have little effect on glucose metabolism.


“I was shocked that we didn’t see sleep restriction alone with minimized circadian disruption result in any adverse metabolic consequences,” Czeisler said.

The second trial, which resulted in the publication of the Metabolism paper, aimed to do the opposite — to test the effect of an irregular sleep pattern alone, with minimal sleep restriction. The researchers also sought to determine whether participants’ glucose metabolism would be impacted by the fat content of their diets.

“We found that the impairment in glucose metabolism was in participants who were exposed to circadian disruption while on a high-fat diet, but not in participants who were exposed to circadian disruption but who consumed a low fat diet,” Zitting said.

Czeisler stressed some of the potential impacts of this study for people who often have irregular sleep schedules.

“We think nothing of the fact that people stay up until three o’clock in the morning on weekends, and then try to get up at at six o’clock in the morning during the week and do this bouncing back and forth, and all these things that we do as if we can just shift our circadian timing and our sleep/wake schedule, and not have any adverse consequences,” he said. “But these data show that that’s not the case.”

Conversely, for those who are unable to avoid an irregular sleep schedule, a lower-fat diet may help reduce long-term health consequences, the paper concluded.

— Staff Writer Paz E. Meyers can be reached at