For Weight Loss, Eating Less After 8 P.M. Might Help

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Increasingly, studies show, getting the right amount of sleep is critical to maintaining a healthy weight. But does it matter when we sleep? A new study suggests that, yes, timing may be key: people who stay up late and sleep in tend to have worse diets, eat more at night and gain more weight, compared with those who go to bed at a reasonable hour.

Previous research in mice has demonstrated that messing with the animals’ sleep and circadian rhythm — the body’s internal clock, which is tied to the 24-hour day’s light-dark cycle — causes them to eat at the wrong times and gain weight. The results were similar in the current study, led by researchers at Northwestern University Feinberg School of Medicine, who said it is the first to examine the relationship between the circadian rhythm, diet and weight in humans.

For one week, 51 people (23 late sleepers and 28 normal sleepers), aged 18 to 71, were asked to record what they ate in a food log — including time and types of food — and to wear a wrist actigraph that monitored their sleep and wake cycles.

On average, the late sleepers went to bed at 3:45 a.m., awoke by 10:45 a.m., ate breakfast at noon, lunch at 2:30 p.m., dinner at 8:15 p.m. and had their final meal at 10 p.m. Normal sleepers, by contrast, were asleep by 12:30 a.m., woke up around 8 a.m., had breakfast by 9 a.m., lunch at 1 p.m., dinner at 7 p.m. and their last snack around 8:30 p.m.

Being a night owl appeared to do more harm than good: for starters, people who stayed up late got less sleep overall (5 hrs. 33 mins. versus 6 hrs. 38 mins. for normal sleepers). Worse, compared with early-to-bed-early-to-risers, late sleepers had a much less healthful diet: they ate more fast food (five meals per week, compared with the normal sleepers’ three), drank more full-calorie soda (4.5 servings per week versus 1.3) and got significantly fewer fruits and vegetables (1.9 servings per day versus 3.4).

Notably, the late sleepers ate roughly the same number of calories per day as normal sleepers (actually, the study found that they ate 248 more calories than the normal sleepers — 2,153 calories versus 1,905 calories — but that difference wasn’t statistically significant). However, they ate a larger proportion of their food later in the day — at dinner or after 8 p.m.

On average, late sleepers’ dinners contained 825 calories, while the normal sleepers’ suppers totaled 630 calories. Those who stayed up late consumed 754 calories per day after 8 p.m., while the normal sleepers ate only 376 calories.

“Human circadian rhythms in sleep and metabolism are synchronized to the daily rotation of the earth, so that when the sun goes down you are supposed to be sleeping, not eating,” said senior author Dr. Phyllis Zee, professor of neurology and director of the Sleep and Circadian Rhythms Research Program at Feinberg, in a statement. “When sleep and eating are not aligned with the body’s internal clock, it can lead to changes in appetite and metabolism, which could lead to weight gain.”

When the researchers looked at participants’ BMI, or body-mass index, they found that caloric consumption after 8 p.m. was most strongly associated with weight, after controlling for length and timing of sleep. The findings, while preliminary, suggest that in addition to the overall quality of your diet, it may be the time of day you’re eating that matters — and your sleep-wake cycle has a lot to with the timing of your meals.

It’s the same result found in studies of mice: when the animals’ circadian rhythms are disrupted, they tend to eat more calories during times they would normally be sleeping, and they gain weight. But when the mice are restricted from eating outside their normally active hours, weight gain is curbed.

These findings could have implications for people who have a hard time losing weight or for shift workers, the authors said. “The study suggests regulating the timing of eating and sleep could improve the effectiveness of weight management programs,” Zee said.

The current study is limited by its small sample size and its reliance of self-reported data, but the researchers said they are planning a series of additional studies to test the findings in a larger population.

The study was published online in the journal Obesity.