Can sleeping late torpedo your diet?

New evidence of how late sleeping undermines healthy eating

Are you a morning person—someone who wakes and rises early, perhaps without the help of an alarm clock? Or do you tend to sleep late, and cherish every extra minute of morning rest? The timing of your sleep may not only matter to your rest itself, but also to your diet. New research indicates that sleeping later is linked to less healthful eating habits, among healthy people who sleep more than six hours a night.

The study, conducted by scientists at Northwestern University’s Feinberg School of Medicine, returned some interesting results about the relationship of sleep timing to diet, physical activity, and weight.

Timing sleep to eat right?

The scientists investigated the relationship of sleep timing to diet and exercise in a group of 96 adult men and women between the ages of 18-50. Everyone included in the study reported getting at least 6.5 hours of sleep a night. None of the participants were shift workers—people whose non-traditional work and rest schedules frequently require them to be active during evening or overnight hours, or otherwise require sleep-wake schedules that deviate from what is broadly regarded as typical.

Over a one-week period, researchers collected measurements of sleep using multiple methods. The participants wore wrist actigraphy bands that collected data about their sleep-wake patterns, including sleep duration, sleep onset (the timing of first falling asleep), and wake times.

To assess biological sleep timing, researchers took a measurement known as dim light melatonin onset, or DLMO. This measurement, conducted in a clinical or laboratory setting, captures the timing of the release of the hormone melatonin under conditions of low light. Melatonin, a hormone triggered by darkness and inhibited by light, plays a critical role in regulating sleep and wakefulness. Increased levels of melatonin help to facilitate sleep, while low levels of melatonin allow for alertness. Assessing the onset of melatonin release under dim-light conditions provides important information about an individual’s circadian timing and sleep-wake cycles.

Scientists also collected data about diet and exercise. Participants kept food journals throughout the weeklong study period, allowing researchers to collect data about calorie consumption as well as eating habits and the nutritional makeup of participants’ diets. Exercise and physical activity were monitored via armbands worn by participants throughout the week.

Their analysis revealed a number of interesting and potentially significant associations between sleep and biological timing and diet. Equally interesting? The points where the investigation revealed a lack of association among these factors.

The study showed significant associations between sleeping late and poor diet. Both later wake times and late biological timing were linked to diets heavier in fast food and lower in vegetables. Late sleepers were also found to consume less dairy in their diets than people who woke earlier. These links between sleep timing and dietary habits were present among both men and women, but were found to be more significant in men.

Sleeping late was also linked to reduced levels of exercise. Participants who slept late tended to be less physically active. However, researchers found no link between biological timing and physical activity. (Remember, biological timing—which researchers evaluated by measuring dim light melatonin onset—involves the expression of individual circadian rhythms to an earlier or later time.)

Despite the links between poor diet and late sleeping, researchers found that late sleepers did not consume more calories than earlier risers. In addition, most of the study participants—including most of the late sleepers—had BMI (body-mass index) that fell within the normal range, which is below 25 for adult men and women. What’s more, late sleepers actually weighed less, on average, than early risers.

Why might sleeping late exert a negative influence over diet, but not over weight or calorie consumption? That’s the question that scientists will seek to answer next. The pursuit of that answer will likely involve investigations of both biological and social forces that affect sleep and circadian function, say researchers.

With diet, sleep timing counts

There is compelling evidence that sleep duration—the amount of sleep we get—has a significant impact on diet as well as on activity levels, weight, and risks for obesity and other metabolic disorders. This study is noteworthy, in part, because it highlights the importance of sleep timing as a factor in diet and exercise habits.

There is other recent evidence suggesting that sleep timing affects diet and weight. Scientists in 2013 found that healthy people who slept on a schedule that combined late bedtimes with short sleep duration were more likely to gain weight than people who went to bed earlier and spent a full night in bed. Researchers attributed the weight gain to additional calorie consumption that occurred during evening hours among participants who stayed up late.

To stay active, get to bed early

Some of the same researchers who conducted the current study also conducted an earlier investigation that looked at the role of sleep and circadian timing to patterns of physical activity. The results from their 2014 study indicate that late sleepers are more sedentary than people with earlier circadian timing and earlier bedtimes—and that these late sleepers also have greater difficulty sticking to an exercise routine. Interestingly, people who considered themselves “night owls” also reported being more sedentary and having trouble exercising regularly—regardless of what time they actually went to bed or woke for the day.

The relationship of sleep to diet, weight, and health is complex and driven by a number of forces, biological and otherwise. What’s becoming increasingly clear is that when it comes to understanding the relationship of sleep to diet and other markers of health, it’s not only the amount of sleep and the quality of sleep, but the timing of sleep that matters.

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