How your bedtime affects your BMI
Too little sleep and too-late bedtimes – at any age – may result in heavier bodies and wider waistlines, new research finds. Body mass index, which measures weight related to height, can rise even for those who get a full night's sleep but on a later schedule. Here's more about the sleep-weight connection, along with suggestions from experts on healthier bedtime routines for you and your family.
Kids Missing Sleep
Babies, toddlers and preschoolers need plenty of sleep, and when they don't get enough, it takes a toll. Dr. Elsie Taveras, chief of general pediatrics at Massachusetts General Hospital for Children, led a study that followed nearly 1,050 children, starting at 6 months old. Every year, up to age 7, mothers reported how long their children slept in a typical 24-hour period, and researchers ranked how often sleep was curtailed.
"We found a very striking relationship when children were chronically getting too little sleep in almost every single outcome you can look at for obesity," Taveras says. "We found higher body mass index, a higher risk of being obese, and at the highest levels of obesity in children, we found higher fat mass."
Kids are sleeping less than their parents did at similar ages and going to bed much later, studies show. "In my own pediatric practice, when we ask about bedtimes, quite often we hear that children are going to sleep at times that are really adult times," she says.
Taveras sees kids as young as 2 years old in her weight-management clinic. Most parents aren't following recommendations to ensure their kids get enough sleep, she says: Some 4-year-olds don't go to bed until 11 p.m. With casual bedtimes at night, but fixed start times for school or day care and for parents to be at work, there's no recovery of lost sleep time.
"We have such strict times for waking up that a 4-year-old has to go to day care in the morning and be up by 6:30," Taveras says. "So there's no way that child is going to achieve the recommended amount of sleep a week that we'd like for them to get."
Metabolism and its delicate balance could be a factor. "In children, some studies show that quality of sleep is very much tied to metabolism," she says. Disrupting that "careful synchronicity" could lead to weight gain in the long run.
Another possible factor: "It has been shown that children who aren't sleeping the recommended amount are also watching quite a bit of screen time," Taveras says. So these kids are seeing more television ads for high-calorie, high-sugar foods with little nutritional value.
The National Sleep Foundation recommends 14 to 17 hours of daily sleep for newborns to 3-month-old babies. Next, it's 12 to 15 hours of daily sleep at ages 4 months to 11 months. Sleep requirements drop to 11 to 14 hours per day for toddlers ages 1 to 2 years. Preschoolers at ages 3 to 5 should get 10 to 13 hours of sleep each day, according to the foundation.
Taveras gives parents the following advice:
- Make sure children get their recommended amount of sleep.
- Keep children on a consistent bedtime routine.
- Maintain similar weekday and weekend bedtimes.
- Remove sleep-robbing electronics and screens from the bedroom.
- Promote good sleep quality for weight management and better behavior.
Young Night Owls
In a new study in the October issue of Sleep, Lauren Asarnow, a doctoral student in clinical psychology at the University of California–Berkeley, and colleagues looked at national data on more than 3,300 adolescents transitioning to young adults – specifically, at their bedtimes and BMI.
"Later bedtime was associated with weight gain," Asarnow says, even after taking typical development for that age group (13 to 32) into account. That held true regardless of the total hours of sleep, she adds: "For example, going to bed at 2 a.m., even if you were getting eight hours of sleep and waking up at 10 a.m., was associated with weight gain."
She offers another example: A woman of average height, about 5 feet 5 inches, and weighing 132 pounds, would gain about 12 pounds or two BMI units by consistently going to bed late during adolescence to adulthood.
Different studies suggest multiple possible causes. One small study found that eating and sleeping at unusual times of the day disturbed people's glucose-insulin metabolism. Another found adults with later bedtimes ate meals later in the day. These later meals were less healthy than those of early-to-bedders, with fewer fruits and vegetables and more calories.
As for her team's findings, Asarnow says, "We see this as potentially very good news. Bedtime is a highly modifiable behavior. So the next question is whether adjusting bedtime affects weight."
To improve sleep, Asarnow suggests making these changes:
- Develop a wind-down routine that includes activities such meditation and yoga.
- Start dimming the lights one to two hours before bedtime.
- Make the bedroom a technology-free zone, from 30 minutes to an hour before sleep time.
- Create weekend bedtimes no more than an hour later than weekday bedtimes to avoid "social jet lag."
Dare to Get Enough Sleep
Last year, a study in the Journal of Clinical Sleep Medicine looked at 225 patients in an urban family medicine center in Akron, Ohio. Dr. Edward Scott and colleagues found significant associations between sleep quality, duration, bedtime stability and obesity.
In a 2010 study comparing sleep quality and BMI among adult twins, shorter sleep was associated with higher BMI. "There's a strong relationship between sleep duration and obesity in both children and adults," says study co-author Dr. Nathaniel Watson, president of the American Academy of Sleep Medicine. "In particular, between short sleep and obesity."
Adults should get at least seven hours of sleep per night, according to revised June guidelines from the American Academy of Sleep Medicine and the Sleep Research Society. When you sleep is also important, Watson says. Nighttime sleep is associated with the release of melatonin – a hormone that regulates sleep-wake cycles – from the pineal gland of brain.
Untreated sleep disorders, like sleep apnea, can dampen people's motivation to exercise. "We do observe that clinically," Watson says. "We do see patients, once they get their sleep apnea treated, have more energy and vitality." That, he says, often creates a positive feedback loop in which people channel that extra energy into a more active lifestyle.
Shift work contributes to the problem, too, creating a mismatch between a person's body clock – their circadian sleep rhythms – and metabolism. A November 2014 study from the University of Colorado–Boulder found night-shift workers tended to burn less energy than other workers during a 24-hour period, putting them at risk for weight gain and obesity. A study of nurses and midwives in Poland, published in July, showed that as workers accumulated more night shifts, their BMIs and hip and weight circumferences increased.
Watson throws down a challenge to readers: "I would just encourage them to spend a two-to-three-week period of time where they go to bed as soon as they start feeling drowsy. And sleep and wake up when they feel rested. In fact, completely sleep-saturate themselves." See how you feel during the day and how effective you are at work, he says, and look at what happens to your mood and well-being. "It can be a transformative event for a person to explore sleep prioritization for a brief period of time," Watson says. "They should view it as a tool for a more effective life."
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