Slug-a-Bed Young Children May Resist Extra Pounds

EVANSTON, Ill., Feb. 8 -- Lack of sleep can weighs heavily on children.

Children who get less shut-eye than their classmates are more likely to be overweight, reported Emily K. Snell of Northwestern University here, and colleagues, in the January-February issue of Child Development.

"Our study suggests that earlier bedtimes, later wake times and later school start times could be an important and relatively low-cost strategy to help reduce childhood weight problems," said Snell, a doctoral candidate in human development and social policy, and colleagues.

In a longitudinal study comparing weight and body mass index in children from the ages of three to 17, they found that "even an hour of sleep makes a big difference in weight status."

"Sleeping an additional hour reduced young children's chance of being overweight from 36% to 30%, while it reduced older children's risk from 34% to 30%," said Snell.

Although several previous studies have linked lack of sleep with overweight and obesity in both children and adults, the authors tried to determine just how the sleep (or lack of it) and BMI are causally related. To evaluate the relationship between sleeping and BMI, the authors first looked at sleep behaviors in children and teens, and then looked for associations between sleep and subsequent BMI and overweight/obesity status.

"Many previous studies control neither for prior weight nor for potentially confounding variables, so we cannot know whether child overweight status contributed to sleep problems or whether child or family characteristics (such as child health status or family schedules) differed in some other way that influenced both sleep behavior and weight," they wrote.

In addition, "little is known about the potentially moderating influences of age on the relationships between sleep and developmental outcomes, although we know that there are age-related shifts in children's sleep needs and the circadian timing of sleep, particularly as children enter the adolescent years."

They drew on data from the first and second waves of the Child Development Supplement of the Panel Survey of Income Dynamics. The survey is a longitudinal study of a representative sample of U.S. individuals and their families.

In 1997, the investigators of that study surveyed 2,394 families who provided information on a total of 3,563 children, known as the first wave (or time 1). In 2002-2003, 2,021 of the families were re-interviewed, resulting in a total of 2,907 child interviews, making up the second wave (or time 2).

Snell and co-authors excluded from their study data on the sleep habits of children who were three years old or younger in 1997, because of potential significant differences in the nature of nighttime sleep between infants/toddlers and older children.

Sleep behaviors were recorded by either older children or the parents/caregivers of younger children. The time diaries included data on bedtime, time asleep and wake time over the course of a weekday and a weekend day.

The main outcome measures were children's standardized BMI and overweight status at the time of the second wave.

In their analysis of sleep behaviors, they found cross-sectional associations between measures of weight and both sleeping very little and having later bedtimes.

Sleeping less than eight hours a night was correlated with higher BMI as well as being overweight during the first wave, while sleeping between 10 and 11 hours a night was correlated with not being overweight, they wrote. In addition, later bedtimes were correlated with being overweight during the first wave. They also found measures of sleep during the first wave to be correlated with BMI and overweight status during the second.

They also found that the more sleep kids got at baseline, the less likely they were to have a high BMI or be overweight during the second wave.

In addition, among younger children (three to eight) later bedtimes were associated with being overweight, while among children nine to 13 earlier wake times were more often associated with overweight.

There were no significant gender-related differences in the effect of sleep on weight, and the authors did not find any associations between lack of sleep and difference in growth rates.

However, they noticed a trend toward children getting less sleep than recommended. For example, they found seven-year-old children were getting fewer than 10 hours of sleep on weekdays, compared with the 10 to 11 hours recommended by the National Sleep Foundation.

Similarly, 16% of adolescents got fewer than seven hours of sleep on weeknights, compared with the eight to nine hours recommended for their age group.

They found that even one additional hour could have a significant positive effect on BMI and overweight status, noting that the chance of a child being overweight during the second wave declined by 5.3 percentage points, from 35.6% to 30.3%, when the average sleep length during the first wave increased from the sample average of 9.9 hours to 10.9 hours.

"If our results represent a true causal relationship between sleep and weight," they wrote, "encouraging parents to put their younger children to bed earlier at night and allowing both younger and older children to sleep longer in the morning, as well as urging school districts to avoid very early school start times for later elementary and middle school aged children, might represent an important and relatively low cost strategy to reduce childhood weight problems."