Children who have later bedtimes at weekends end up being fatter

Letting schoolchildren stay up a bit longer at the weekend may seem like a harmless treat.

But children whose bedtime is later at weekends than during the week tend to be heavier, with larger waistlines, a study suggests.

That is likely due to ‘social jetlag’ – the shift in someone’s body clock caused by varying bedtimes which may also affect how efficiently they process food.

Researchers looked at 10,245 children aged nine to 12 in Finland who were asked what time they went to bed on school nights and non-school nights.

The results show the average child going to bed two hours later at the weekend had a waist circumference about an inch (2.4 centimetres) larger.

This was based on a child 148cm (4ft 10ins) tall.

An average child whose bedtime was two hours later would have a body mass index (BMI) which was 0.32 higher than children with consistent bedtimes, the results suggest.

Dr Sohvi Lommi, co-author of the study from the Folkhälsan Research Centre in Finland, said: ‘Parents should pay attention to bedtimes and try to keep a regular routine at weekends.

‘These results suggest that children may put on more weight if their bedtimes move around too much.’

The study authors conclude: ‘Promoting early bedtimes, especially on weekends, should be considered in obesity prevention among school-aged children.’

They add: ‘Consistent bedtimes between non-school and school days may be achieved in two ways: either by allowing later school start times or pursuing earlier bedtimes on non-school days.

‘Of these the former seems more feasible for adolescents and their families.’

Around one in eight children go to bed at least two hours later at the weekend, based on the new study.

That likely means sleeping later the next day, with no shouting parent or alarm clock waking them up.

Experts believe this difference in sleeping and waking times throws off someone’s internal body clock – which is called social jetlag because it feels a bit like jetlag from flying to a country with a different time zone.

The new study provides evidence supporting the idea that social jetlag causes people to put on weight, as children with later weekend bedtimes had a higher BMI, standardised for their age and sex, and a higher waist circumference for their height.

Among the 10,245 children looked at, who took part in Finnish study on teenage health, around half provided further information two and a half years later, when their parents re-measured their weight, height and waist circumference.

At this point, a two-hour later weekend bedtime at the age of 11 was linked to a waistline which was almost a quarter of an inch (0.6cm) larger than seen in children with consistent bedtimes.

Bedtimes which jumped around were linked to a higher weight and larger waistlines in children regardless of how long they slept.

The average child in the study went to bed at 9.30pm on a school night and 10.30pm on a non-school night.

The study found children who spent more time in front of television and computer screens delayed their bedtimes by longer at weekends.

However the researchers note that children self-reported their bedtimes, which could have made them inaccurate, and the bedtimes may have changed over time.

The study, published in the Journal of Sleep Research, was reported by New Scientist magazine.