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Shift workers may have higher rates of death from heart disease, stroke, diabetes, dementia, cardiovascular disease, and cancer. Graveyard shift, indeed! But, is it just because they’re eating out of vending machines, or not getting enough sleep? Highly controlled studies have recently attempted to tease out these other factors by putting people on the same diets, with the same sleep, but just at the wrong time of day. Redistributing eating to the nighttime resulted in elevated cholesterol, blood pressure, and inflammation. No wonder shift workers are at higher risk; shifting meals to the night in a simulated night shift protocol turned about a third of the subjects effectively prediabetic in just 10 days. Our bodies just weren’t designed to handle food at night.

Just as avoiding bright light at night can prevent circadian misalignment, so can avoiding night eating. We may have no control over the lighting at our workplace, but we can try to minimize overnight food intake, which has been shown to help limit the negative metabolic consequences of shift work. When we finally do get home in the morning, though, we may disproportionately crave unhealthy foods. In this experiment, 81% of participants in a nightshift scenario chose high-fat foods such as croissants out of a breakfast buffet, compared to just 43% of the same subjects during a control period on a normal schedule.

Shiftwork may also have people too fatigued to exercise, but even at the same physical activity levels, chronodisruption can affect energy expenditure. Researchers found that you burn 12-16% fewer calories while sleeping during the daytime compared to night. Just a single improperly-timed snack can affect how much fat you burn every day. Study subjects eating a specified snack at 10 am burned about 6 grams more fat from their body than on the days they ate the same snack at 11pm. That’s only about a pat and a half of butter’s worth, but it was the identical snack––just given at a different time. The late snack group also suffered about a 9% bump in their LDL cholesterol within just 2 weeks.

Even just sleeping in on the weekends may screw up our metabolism. “Social jet lag” is the discrepancy in sleep timing between our work days and our free days. From a circadian rhythm standpoint, when we go to bed late and sleep in on the weekends, it’s as we flew a few time zones west on Friday evening and fly back Monday morning. Travel-induced jet lag goes away in a few days, but what might the consequences be of constantly shifting your sleep schedule every week over your entire working career? Interventional studies have yet put it to the test, but population studies suggest that those who have at least an hour of social jet lag a week (which may describe more than two-thirds of people) have twice the odds of being overweight.

If sleep regularity is important, what about meal regularity? The importance of regular meals at roughly the same time every day was evidently emphasized by such luminaries as Hippocrates and Florence Nightingale, but wasn’t put to the test until the 21st century. A few population studies had suggested that those eating meals irregularly were at a metabolic disadvantage, but the first interventional studies weren’t published until 2004. Subjects were randomized to eat their regular diets split up into 6 regular eating occasions a day, or 3 to 9 a day in an irregular manner. Researchers found that eating an irregular pattern of meals every day can cause a drop in insulin sensitivity and cause cholesterol levels to rise, and reduce the calorie-burn immediately after meals in both lean and obese individuals. They ended up eating more, though, on the irregular meals. And so, it’s difficult to disentangle the circadian effect. The fact that overweight individuals may overeat on an irregular pattern may be telling in and of itself, but it would be nice to see such a study repeated using identical diets to see if irregularity itself has metabolic effects. And, just such a study was published in 2016.

During two periods, people were randomized to eat identical foods in a regular or irregular meal pattern. During the irregular period, people had impaired glucose tolerance––meaning higher blood sugar responses to the same food, and lower diet-induced thermogenesis––meaning burned fewer calories to process each meal. The difference in thermogenesis only comes out to be about 10 calories per meal, and indeed there was no difference in weight changes over the two-week periods. But diet-induced thermogenesis can act as a satiety signal. The extra work put into processing a meal can help slake one’s appetite. And indeed, lower hunger and higher fullness ratings during the regular meal period could potentially translate into better weight control over the long term.

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