Daytime eating maintains circadian alignment, prevents glucose intolerance despite mistimed sleep

What are the effects of eating during the nighttime instead of the daytime? New research, focused on a simulation of night shift work, may hold implications for people eating at atypical times -; like those experiencing jet lag, circadian rhythm sleep disorders, or who tend to sleep late during the weekends.

A study by investigators from Brigham and Women's Hospital indicates eating during the nighttime can lead to glucose intolerance, while limiting meals to the daytime can help regulate bloodstream glucose levels. Nighttime eating appeared to cause a misalignment between the body's central and peripheral circadian "clocks" -; natural timekeepers that regulate physical, mental and behavioral changes over a 24-hour cycle. The researchers' results are published in Science Advances.

These results indicate that meal timing was primarily responsible for the reported effects on glucose tolerance and beta-cell function, possibly due to the misalignment of central and peripheral 'clocks' throughout the body. While the central circadian 'clock' was still on Boston time, the endogenous circadian glucose rhythms suggest that some peripheral 'clocks,' as perhaps those in the liver, had dramatically shifted to a time zone in Asia."

Frank A.J.L. Scheer, PhD, MSc, Study Co-Corresponding Author, Department of Medicine and Director of Medical Chronobiology Program in Division of Sleep and Circadian Disorders, Brigham and Women's Hospital

Glucose intolerance leads to high glucose levels and often precedes type 2 diabetes mellitus (T2DM), a condition in which the body is less able to absorb sugar from the bloodstream into its tissues. T2DM is prevalent among night shift workers who typically sleep during the daytime and eat during the nighttime. Previous laboratory studies showed increased levels of blood glucose in both non-shift workers and shift workers who underwent simulated night work, said co-corresponding author Sarah L. Chellappa, MD, PhD, formerly of the Medical Chronobiology Program and now of the Department of Nuclear Medicine at the University of Cologne in Germany. They added that shift workers, while frequently exposed to mistimed meals, are not necessarily "immune" to their adverse effects.

This study, a randomized controlled trial, involved 19 healthy young participants who underwent a 14-day controlled laboratory protocol. During the study, participants stayed awake for 32 hours in a highly controlled, dimly lit environment, where they kept constant body posture, consumed identical snacks every hour, and had no time cues -; conditions that are part of a constant routine protocol.

After that, the participants underwent simulated night work and followed one of two eating schedules: one group ate during the nighttime to simulate a schedule typical among night shift workers, while the other group ate during the daytime, thus aligning their meal schedule to the ~24-hour cycle of the central circadian "clock." Subsequently, participants followed a second, 40-hour constant routine protocol to assess the aftereffects of the meal schedules on their endogenous circadian rhythms.

According to the research, participants who ate during the nighttime showed increased blood glucose levels, while those who ate only during the daytime showed no significant changes. In addition, nighttime eating decreased pancreatic beta-cell function compared to no observable changes in those eating only during the daytime.

Beta cells produce insulin, a hormone that escorts glucose into body tissues. Furthermore, nighttime eating caused a misalignment between the central circadian "clock," estimated from the endogenous circadian rhythm in core body temperature, and the endogenous circadian glucose rhythms. In stark contrast, these rhythms remained aligned when participants ate meals only during the daytime despite their mistimed sleep.

"Of the participants studied, those with the biggest disruption of their circadian system -; here quantified as the misalignment between their central circadian 'clock' and their endogenous circadian glucose rhythms -; showed the largest impairment of glucose tolerance," said Scheer.

The study's take-home message indicates that daytime eating, despite mistimed sleep, maintains internal circadian alignment and prevents glucose intolerance. The authors note that more research needs to be conducted to find practical interventions to implement daytime eating in real-life shift workers.