KEY POINTS:
A study of New Zealand seven-year-olds has found that sleeping fewer than nine hours a night is associated with being overweight or obese, even after accounting for time spent watching television, and physical exercise.
The study, published yesterday in the scientific 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.
"The study is important from the perspective of providing another means of preventing obesity," said Ed Mitchell, the senior author and a professor of child health research at Auckland University.
"At least in New Zealand - and it needs to be confirmed in other age groups - this seems to be an important factor."
Sleep time did not affect IQ scores or measures of attention-deficit/hyperactivity disorder, but children who averaged fewer than nine hours sleep were significantly more likely than the others to be overweight.
On average, the children stayed awake for 48 minutes after they went to bed, and slept about half an hour longer on weekdays than weekends.
They slept the least in the summer: 40 minutes longer on winter nights, 31 minutes longer in the autumn and 15 minutes longer in the spring.
Having a younger sibling cost a seven-year-old an average of 12 minutes of sleep per night.
The findings are similar to a study published by US researchers in November. They worked out that every additional hour of sleep a night for third-graders (aged 8 or 9) reduced their chances of being obese in sixth grade (aged 11 or 12) by 40 per cent.
Sleeping more than nine hours and 45 minutes lowered the risk significantly.
A possible cause was that sleep deprivation produced more ghrelin, a hormone that promotes hunger, and less leptin, a hormone that signals fullness.
Dr Julie Lumeng of the University of Michigan, who led the research, said the explanation could be simpler: Tired kids are less likely to exercise and more likely to sit on the couch.
Time for bed
The average NZ seven-year-old
* Stays awake for 48 minutes after going to bed.
* Sleeps about half an hour longer on weekdays than weekends.
* Sleeps least in summer and most in winter.
* Loses 12 minutes of sleep each night if he or she has a younger sister or brother.
