Teenaged girls who stay up late every night could pay a price in added pounds, new research shows.
There could even be greater ramifications for girls' health, with risks for "cardiometabolic" issues -- such as heart disease and diabetes -- rising with later bedtimes, the researchers said.
A similar trend was not shown for boys, although the research team stressed that that might be because the number of boys in the study was too low.
However, for teen girls at least, the researchers "found that 'night owls,' teenagers who prefer to go to bed late but have to get up early for school, had higher waist circumference and greater abdominal fat deposition (adiposity) than the 'morning larks,' those who prefer to go to bed early and get up early to begin their day," said study senior investigator Dr. Elsie Taveras. She directs general academic pediatrics at the Massachusetts General Hospital for Children, in Boston.
Prior studies have linked insufficient sleep in teens with obesity and poor cardiometabolic health, the study authors noted. However, less attention has been paid to the impact of when kids get their shut-eye.
The new research included data on more than 800 kids, aged 12 to 17, who were enrolled in a study that for 20 years has tracked the health of growing children.
Taveras' group looked specifically at data on whether teens were "early birds" or "night owls," and on something called "social jet lag" -- differences in when teens went to sleep on school nights versus non-school nights.
Teen girls who tended to go to sleep later had an average waistline measurement that was about a quarter-inch higher than that of "early birds," the findings showed. They also had a slightly higher amount of body fat, on average, according to the report published online Sept. 16 in JAMA Pediatrics.
The study couldn't prove that bedtimes helped cause added weight gain, but one sleep specialist wasn't surprised by the findings.
"We have known for some time that when humans are sleep-deprived, they tend to gain weight; this is likely an effect of sleep deprivation on two hormones that regulate appetite," said Dr. Steven Feinsilver. He directs the Center for Sleep Medicine at Lenox Hill Hospital in New York City.
And as Taveras noted in a university news release, "beyond [sleep] quantity and quality, timing is a vital component of sleep because it determines if an individual's circadian clock -- the internal sleep/wake schedule -- is synchronized with the rhythms of their daily activities."
She pointed out that "this is particularly important to adolescents whose evening preferences and academic demands often result in irregular sleep schedules that may cause circadian misalignment."
The study authors stressed that maintaining a consistent bedtime seven days a week is crucial. Doing so can cut social jet lag, and perhaps also minimize risks for obesity and poor cardiometabolic health.
Dr. Michael Grosso, chief of pediatrics at Northwell Health's Huntington Hospital, in Huntington, N.Y., agreed.
The study findings "make sense," he said, "and may form the basis for good medical advice. The take-away: get enough sleep, and keep bedtimes regular."
For her part, study lead author Elizabeth Cespedes Feliciano advised parents to "encourage consistency in their children's sleep schedules and their bed and wake times, as well as improvements in their sleep hygiene by limiting electronic media and caffeine use in the evening." Feliciano is a research scientist in the Kaiser Permanente Northern California Division of Research.
Taveras said that schools can also help by having later morning start times. Schools can also make it easier for students to have time during the day to complete academic activities that might otherwise be done very late at night.