Studies have indicated a relationship between short sleep duration and obesity in children and adults. The cause of the elevated weight in short sleepers is not known. There is relatively little research done on the relationship between the timing of sleep, weight, and activity patterns. In the Journal Sleep is a study about this topic. The researchers classified sleep timing as early to bed / early to rise, early to bed / late to rise, late to bed / early to rise, and late to bed / late to rise. They studied Australian children ages 9 - 16 years old, and surveyed their sleep habits, physical activity, and measured their weight, height, and waist circumference.
The results showed children who were late to bed / late to rise were 1.5 times more likely to be obese, 1.8 times to have too little physical activity, and 2.9 times more likely to have excessive use of computers and television (a.k.a. screen time) than the early to bed / early to rise group. Interestingly, the greatest differences in the above measurements were between the late to bed / late to rise and early to bed / early to rise groups, which slept the same amount per night. There was little difference in early to bed / late to rise and late to bed / early to rise, which had an average total sleep time difference of over 2 hours per night. This is contrary to the prior studies that suggest sleep duration is linked to obesity. The results from this study imply that it is the timing of the sleep, not the sleep duration, that is associated with obesity.
Some advocate that adolescents be allowed to stay up late and sleep in, including later school start times. However, this might make the obesity problem even worse, according to the study results. If anything, this study suggests that children should be put to bed earlier and gotten up earlier.