Does lack of sleep make children overweight? Here's what the science says

Much is said about today’s children not getting enough sleep, a problem blamed on lax bedtimes, and excessive evening computer and mobile phone use. Plus ça change. A hundred or so years ago, it was just as bad – but then excessive homework was a culprit.

In 1884, for example, the British Medical Journal reported that Crichton-Browne, a renowned psychiatrist of that era, testified to parliament that:

I have encountered many lamentable instances of derangement of health, diseases of the brain, and even death resulting from enforced evening study in young children, with the nervous excitement it so often induces … it implies a maximum of effort with a minimum result.

Later, in 1908, the persistent and now knighted Sir James Crichton-Browne, in his presidential address to the Child Study Society, bemoaned that “the evil of insufficient sleep in children is widespread”.

He was responding to a talk by educational pioneer Alice Ravenhill who had conducted a three year investigation into the sleep of elementary school children. As part of this, 10,000 forms had been issued of which 6,180 “were properly filed up, and gave particulars as to 3,500 boys and 2,680 girls”.

According to this data, the average three to five-year-old was getting 10.75 hours of sleep a night, while 13-year-olds were getting eight. Both Ravenhill and James had been advocating 14 hours’ sleep for the former group and 10.5 hours for the latter, which in his words represented “a loss equivalent to one night in four in the youngest children, and one night in five among those of intermediate ages”.

Sleeping beauties?

Soon afterwards, in 1913, a meticulous study on US schoolchildren by Lewis Terman and Adeline Hocking from Stanford University, reported children getting an average of 11 hours at age six and nine hours at 13. Terman was a renowned educational psychologist and their insightful conclusions are quite apposite today:

Sleep is but one of the many needs of children, and it is foolish to make it the scapegoat for all kinds of physical and mental evils as hygienists have so often done. It is possible that the quantity of sleep is less important than its quality, and that when disturbances of the latter occur they are more likely to be the effect of ill-health than its cause … sleep cannot be accurately measured in units of time alone…

But what about now? Well, two important findings were published in 2012. A survey of 11,000 UK children, by Peter Blair and colleagues from Bristol University, reported that the average sleep duration for six and ten-year-olds was 11.3 and 10.5 hours respectively, with the emphasis on wide natural variations of sleep at all ages.

Both values are rather longer than those reported by Ravenhill.

Meanwhile, Lisa Matricciani and colleagues reported on children’s sleep trends between 1905 and 2008. The study covered 20 countries and a total of 690,000 five to 18-year-olds. It concluded that, overall and on average, primary school children are getting 30 minutes’ less sleep today than they were 100 years before, but that children in Australia and the UK are reversing this trend by sleeping for about an hour longer than they were in the early 20th century. In mainland Europe, the US and Canada, however, children are sleeping for an hour less.

Again, it is emphasised that there are large, natural variations in sleep duration across all ages, and one must not be too prescriptive.

What about obesity?

Various statistically significant findings link short sleep with obesity in children, perhaps implying that “short sleep” contributes to the “obesity epidemic”. But much of this is easily misinterpreted and even of minor clinical concern. That is, statistical and clinical significances are not always synonymous.

Indeed, short sleep and obesity may well have the same underlying cause, such that longer sleep will not overcome the obesity. Another concern is that studies often split children into those sleeping either more or less than (usually) an arbitrary ten hours a night, including those far below this threshold, who might indeed be somewhat obese, whereas nine hour sleepers are not.

Some research points to a doubling of obesity in short sleepers. But while this might seem alarming, the values are often small. One study, for example, found that that 5.4% of short sleepers (defined as those sleeping less than ten hours a night) were obese, while only 2.8% of those sleeping over ten hours were. This is indeed a doubling, but the actual difference is only 2.6%.

Sleep durations in overweight versus normal weight children are also are liable to overstatement. Research has found that while normal weight 12-year-olds sleep an average of 9.02 hours a night, overweight 12-year-olds sleep for 8.8 hours. This is a statistically significant difference, but it also only amounts to 14 minutes.

The Avon study of 7,758 UK children monitored from birth until age seven found that 9.2% of the boys and 8.1% of the girls had become obese. Although short sleep seemed to be a factor in the obesity, so was parental obesity, and watching television for more than eight hours a week. And by inverting the percentages, 90.8% of the short sleepers were of normal weight, compared with 91.9% for the others. Hardly a great difference.

I am not critical of the research in such studies, only of the interpretations, as sleep has a much smaller – and only a slowly developing – impact on body weight than is often assumed.

Besides, there are also findings, attracting less interest, that report no link between short sleep and body weight in children. One example is the detailed US National Survey of Children’s Health, conducted in 2003. It looked at 81,390 six to 17-year-olds, and after socio-demographic variables were taken into account, the authors concluded “that the role of insufficient sleep in the childhood obesity epidemic remains unproven”.

Clearly, better diet and more physical exercise are much more likely to help maintain normal body weight in children, as well as having other health benefits. Sleep, as a cause here, seems to be a bit of a red herring.