For women, inconsistent bedtimes linked to body fat, weight gain
PROVO — People who go to bed and wake up at vastly different times or short themselves on sleep have less-than-dreamy results when it comes to controlling body fat, according to new research from BYU.
High-quality, consistent sleep influences the ability to maintain a healthy body weight, researchers found in results published online in the American Journal of Health Promotion. The Brigham Young University researchers said that women who change the time they get up or go to bed by more than 90 minutes during the week are more prone to body fat than those who are more consistent in their sleep.
"An interesting findings was that the pattern of sleep matters — when you go to sleep and when you get up," said Bruce Bailey, the exercise science professor who led the study of more than 300 women attending two big universities in the West.
Getting up at about the same time every morning is particularly related to lower body fat and lower Body Mass Index, but going to bed the same time matters as well. The effect was stronger on body fat than on BMI, he said.
The best sleep-duration range was between 6.5 and 8.5 hours a night, and Bailey said 8 to 8.5 hours a night was the "sweet spot" — but the consistency and quality of the sleep was even more important. "We all have an internal circadian clock and if you don't go to sleep at a consistent time, you mess with that internal clock. It has an impact on different physiological aspects, like hormones related to satiety and activity patterns," he said.
Bailey said women in the study who got more than 8.5 hours of sleep had really high body fat and it's possible something else was going on with them that affected their weight and how much sleep they got. Those who slept fewer than 6.5 hours also had more body fat than those who fell into that desirable mid-range of sleep.
"It's not just how much sleep, but the quality of the sleep, and several factors can be used to determine that," said Larry Tucker, an exercise science professor and co-author of the study.
The researchers used tools to make sure their assessments were accurate. The participants, 330 women ages 17 to 26, had their body composition measured and were asked to fill out a sleep log. They wore an activity monitor — an accelerometer that is "extraordinarily precise," said Tucker — for a week, both day and night. It let researchers objectively determine the quality and duration of their sleep. The better the quality of the sleep, the healthier the weight, they found.
"Some people sleep like a log and don't move, they sleep very peacefully, and others are not really sleeping, constantly tossing and turning and waking up," said Tucker. The device also told the researchers when the individual went to bed, although they recorded the time on their log, as well.
The participants' age range, 17 to 26, was selected because "young adults tend to have more unusual sleep patterns than others," said Tucker. "By the time we're older, we learn the value of sleep and usually adapt so we can get enough. At this age, they still have the strength of youth and may feel like they can skip some sleep."
Women were excluded from the study if they were not able to engage in moderately intense activity, were dieting to lose weight, were pregnant or nursing or had a known metabolic disease. Certain medications also excluded them.
Bailey said there's no reason to think the findings on sleep and weight would be different with men or older age groups. "There are a number of other studies that show sleep is related to body weight," he said. "There's enough evidence that's similar."
Other sleep studies
Tucker said that other studies have shown sleep is important for many reasons, including brain function. "It's unbelievable how important getting enough sleep and being consistent in sleep is. The body doesn't function well if we don't get regular sleep and enough sleep. People who don't sleep consistently end up with problems of all sorts," he said, "not just cognitive function but personality function, too."
Several studies have linked sleep to weight. Last year, the University of Chicago showed that fat cells become less sensitive to insulin when sleep duration is reduced. A Chicago study of men noted that those who are sleep-deprived crave and consume calorie-dense foods, increasing their weight.
While a University of Colorado study found that people who stay up very late burn more calories, that study also demonstrated that night owls also eat enough more to actually gain weight despite the quicker metabolism, compared to people who sleep for around nine hours a night. The proverbial midnight oil appears to stick to the belly and thighs. And Stanford research showed that those sleeping fewer than eight hours had more body fat — and the fewer the hours slept, the higher their weight.
International studies also back up a sleep-weight connection. For example, a Czech study conducted by researchers at the Institute for Clinical and Experimental Medicine concluded that the best body weight and most healthy BMI numbers were associated with seven hours of sleep.
Sleep needs vary among individuals, but the Centers for Disease Control and the National Sleep Foundation both recommend adults get between 7 and 9 hours per night, while the National Institutes of Health sets the goal at 7 to 8 hours a night. Adolescent girls are supposed to get between 8 and 10 hours.
Health experts refer to sleep hygiene, and Bailey is no exception. He pointed out a number of things that people can do to make sure they get good quality sleep, including "the nature of the bedroom and what you do in your bedroom. Is it a cue for sleep or are you using it for other things?" Factors like temperature, the bedroom's lighting and how active a person is during the day "all have some influence on the quality of your sleep," he said.
Other BYU coauthors are James LeCheminant, also an exercise science professor, and statistics professor William Christensen.
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