We know that what we eat matters, but how about when we eat? 

As the adage goes, timing is everything. This certainly applies to eating. By aligning meals with our internal circadian rhythms — the system that orchestrates sleep and wake times, hormone production, metabolic processes, and more — data strongly suggest that we can reduce the health risks associated with eating at the wrong biological time.

As an example, night shift work is omnipresent in contemporary society, and at this point it is inconceivable for humans to exist without it. From truck drivers to hospital and factory workers, our 24/7 society is interwoven with and relies heavily on these nontraditional work schedules. The flipside is that night shift workers are at high risk for illness, including diabetes, heart disease and depression.

Why is this?

Our research and the work of others has shown that circadian misalignment — the misalignment between the central circadian “clock” located in the brain’s suprachiasmatic nucleus and our daily behaviors — negatively affects metabolism. As night workers typically experience circadian misalignment, this may explain increased health risks.

Is there a way to prevent such health risks?

We have been seeking to find out. We ran a clinical trial with 19 healthy young participants (7 women). The participants engaged in a ‘constant routine’ protocol that researchers use to study internal circadian rhythms. The participants remained awake for many hours in a dimly lit environment, had a constant body posture, identical snacks every hour and no time cues. After we established this baseline, they participated in a simulation of night shift work with one of two meal schedules: one group ate during the night, which is very common among night workers, and the other group ate during the day, thereby keeping their meal schedule aligned with their central circadian rhythm. They then engaged in a second constant routine to study the aftereffects of nighttime and daytime eating on internal circadian rhythms. We found that nighttime eating impaired glucose tolerance — that is, higher blood sugar levels after eating — a risk factor for diabetes. On the other hand, restricting meals to the daytime, even while maintaining night shift work, prevented this increase in blood sugar levels.  

The human body tells a fascinating story, full of Sherlockian mysteries. Take the benefits of daytime eating demonstrated in our study: the mechanisms are complex and multilayered.

  • One idea is that nighttime eating while on the night shift increases blood sugar levels because the beta cells in the pancreas are not prepared to work properly. Beta cells produce insulin, the well-known hormone that escorts sugar into body tissues (liver, muscles, and others). If these cells are not working properly, sugar levels will increase greatly after eating. That is exactly what we found.
  • Another idea is that during night work there is a mismatch of daily behaviors with the central circadian clock. As a result, night workers end up eating at the wrong biological time, when our body cannot cope efficiently with food ingestion. Again, that is what we found.
  • Night work may cause internal circadian misalignment, where different clocks in the body get out of sync. Instead of working as an ensemble, one clock may direct peak functioning at noon while another is focused on 6 p.m., and so on. As a result, the body receives mixed messages that create metabolic mayhem. Indeed, that is also what our study showed: a misalignment among different internal circadian rhythms.

Now, if you connect these three findings, you can appreciate how daytime eating (with clocks in synchrony) would keep blood sugar levels under control despite the night work.

Although we did not study real-life shift workers in their natural work environment, our lab study provided an important breakthrough in the fight against the serious health risks of night shift work. The take-home message is that when you eat does affect health outcomes. With that in mind, discouraging nighttime eating will likely have both immediate and long-term health payoffs.


Sarah L. Chellappa MD, PhD and Frank A.J.L. Scheer PhD are chronobiologists from the Division of Sleep Medicine, Department of Medicine at Harvard Medical School, who study the human body clock and strategies for healthier shift work.