Getting enough sleep is important for staying slim, but new research from Brigham Young University indicates that sleeping too late could you make you fatter.
A bedtime or wake-up time change of just 90 minutes or more could be enough to affect body fat, according to a new study published in the American Journal of Health Promotion.
That's the difference between getting up at 7 a.m. during the week and sleeping in until 9 a.m. on the weekends.
"I think we have our circadian rhythms, this internal clock, and when we consistently get up at different times it makes it difficult for our body to get into a rhythm," said Bruce Bailey, lead author and BYU exercise science professor.
His study found young adult women whose sleep consistency varied by more than an hour and a half during a seven-day period had higher body fat than those whose patterns changed by less than 60 minutes. Wake times appeared to have a larger effect than bedtimes.
That could be because women with more consistent routines lead healthier lifestyles, or because sleep affects hormones related to weight management, particularly those that tell you when you're full.
If not sleeping enough is bad, too much sleep could also be unhealthy: The researchers also found that women who got more than 8.5 hours of sleep had similar body fat levels to those who slept less than six hours, though the number of longer sleepers in the study was relatively small.
Sleep duration, though, didn't appear to matter as much as consistency and sleep quality, or how much time in bed is actually spent snoozing.
To improve sleep quality, Bailey recommended keeping the room cool, quiet and dark, exercising during the day, and using beds only for sleeping.
The study included 330 women from BYU and Utah Valley University. Researchers recruited subjects aged 17 to 26 and measured their weight and body fat, then asked the women to keep their normal routines and used activity monitors to monitor them for a week.
"The message, I don't think, is actually all that new," Bailey said. "This is unique in the way it's applied to body fat and looking at [weight management]."