A message to night owls: There's news that your bedtime -- and those late-night snacks -- may be preventing you from dropping those stubborn extra pounds. A recent study took on an important, and under-examined, aspect of the sleep-weight loss connection: how the timing of sleeping -- and of eating -- can affect weight. Researchers at Northwestern University examined the effects of sleep timing on diet and body-mass index (BMI), and found that late bedtimes and late mealtimes can lead to less healthful diets and to weight gain.
A group of 52 adults -- 25 women and 27 men -- spent seven days keeping food logs and having their sleep and waking activity measured by a wrist sensor. The researchers divided participants into two categories of sleepers:
- "Normal sleepers" reached the midpoint of their night's sleep before 5:30 a.m. These sleepers were asleep by shortly after midnight, and woke around 8 a.m. Among the study group, 56 percent were normal sleepers.
- "Late sleepers" reached the midpoint of their nightly sleep after 5:30 a.m. They went to sleep in the middle of the night, well after midnight, and woke in the mid-to-late morning. Among the study group, 44 percent were late sleepers.
Researchers tracked the eating habits of these two types of sleepers through the information provided to them from the participants' food logs. Not surprisingly, "normal sleepers" and "late sleepers" were on very different schedules, in terms of when they ate throughout the day:
- Normal sleepers ate breakfast by 9 a.m., lunch at 1 p.m. and dinner at 7 p.m., on average. These sleepers reported being finished with eating for the day by 8:30 p.m.
- Late sleepers reported eating their first meal of the day at about noon. They ate again in the middle of the afternoon, and did not eat dinner until after 8 p.m. Late sleepers did not finish their eating for the day until 10 p.m., on average.
What were the consequences for sleeping later and eating later? Researchers found that late sleepers suffered across the board, in terms of the quality and quantity of both their daily sleep and eating:
- Late sleepers slept less overall than normal sleepers -- an average of more than an hour less per night.
- Late sleepers consumed more calories at dinnertime than normal sleepers. They also consumed significantly more calories after 8 p.m.
- Late sleepers had poorer quality diets than normal sleepers -- they ate more fast food, drank more sugar-laden soda, and ate fewer vegetables.
These habits of late sleepers -- sleeping less, going to sleep later in the night, and eating more after 8 p.m. -- were all found to be associated with a higher body mass index. Among these habits, eating after 8 p.m. was the strongest predictor of a higher BMI. What does this mean? It's not just what you eat, but when you eat, that can affect your ability to lose weight. And your sleep habits can have a significant influence on the timing of your eating, as well as on how much you eat.
Previous studies using mice have shown results similar to these. Mice whose sleep-wake cycles were disrupted by exposure to light during their normal sleeping hours ate more and gained more weight than mice whose cycles remained intact. This new research has given us a strong start in examining this aspect of the sleep-weight relationship in humans. More investigation is needed, but these results do align with much of what we already know about sleep and weight:
- When deprived of sleep, the body undergoes a shift in hormones, generating more of the hormones that boost appetite, and less of the hormone that signals a feeling of fullness.
- Sleeping less than six hours per night (or more than eight hours per night) over an extended period of time makes weight gain more likely, according to a long-term study.
- Lack of sleep significantly reduces the body's ability to burn calories during waking hours.
This is not just an issue for adults. Kids -- teens especially -- are particularly prone to late and erratic bedtimes, midnight snacks, and a general lack of sufficient sleep, which can lead to weight gain. We know that adolescents require more sleep than adults, but they're too often making do with much less sleep than they need to function well, and with obesity an increasingly common health problem for children as well as adults, this new research provides yet another reason kids and teens need the structure of a sleep schedule -- one that includes curbing eating at a reasonably early hour. Weight problems that develop during childhood and adolescence can have long-term consequences that affect health for a lifetime. Let's give our kids a healthy start by helping them develop the skills they need to eat and sleep well.
Come to think of it, that's advice we can all take to heart.
Michael J. Breus, PhD
The Sleep Doctor™
Everything you do, you do better with a good night's sleep™
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