A study of 7-year-olds has found that sleeping less than nine hours a night was associated with being overweight or obese, even after accounting for amounts of television watching and physical exercise.

The study, being published in the journal Sleep, also found that short sleep duration was associated with mood swings. The researchers had followed the subjects — 519 children in New Zealand — since birth, making periodic health and developmental assessments and interviewing their parents.

Sleep time did not affect IQ scores or measures of attention-deficit/hyperactivity disorder, but children who averaged less than nine hours' sleep were significantly more likely than the others to be overweight. Using sleep monitors, the scientists discovered some other patterns in the 7-year-olds. On average, the children stayed awake for 48 minutes after they went to bed, and slept about a half-hour longer on weekdays than weekends.

They slept the least in summer: 40 minutes longer on winter nights, 31 minutes longer in the fall and 15 minutes longer in spring.

Science Times