Teenagers who stay up later are more likely to be fat, research has suggested – especially if they're girls.
Scientists studied the waistlines and sleeping patterns of hundreds of 12 to 17-year-olds.
They uncovered a link between whether someone was an early bird or a night owl and how large their waist was.
Youngsters with the worst 'social jet lag' – often caused by being a night owl – were most likely to be obese, results showed.
The scientists said their weight may increase because interrupting their sleep led to unhealthier lives in general, them being less active and using their phones more.
The research adds to past findings that staying up late may bring negative effects such as a higher risk of depression, a shorter attention span or an early death.
In a study, teenagers who had bigger differences in their sleep patterns in the week versus the weekend were more likely to be obese (stock image)
Researchers from Massachusetts General Hospital for Children and Harvard Medical School carried out their study on 804 children.
They assessed whether the children preferred being up early or late, the differences between their weekdays and weekends, and measured their waistlines.
Social jet lag is how things you're forced to do – work or school – can interrupt your preferred sleep-wake cycle.
It can be judged by comparing the mid-point of someone's sleep at the weekend to the mid-point in the week.
If someone would naturally get up late at the weekend but has to get up early in the week, they are deemed to have greater social jet lag.
'Large variability in sleep patterns across the week can disrupt normal physiology, resulting in obesity and [heart] risk,' said Dr Elizabeth Feliciano.
'Our study supports the importance of biological clocks in influencing obesity risk.'
In Dr Feliciano's study, each hour earlier a child had to get up in the week led to a 62 per cent higher risk of them becoming obese.
So if, for example, they had a one per cent chance of obesity to begin with, this rose to 1.6 per cent if the middle of their day was brought forward an hour in the week, or 2.2 per cent for two hours.
WHAT HAPPENS IF YOU DON'T SLEEP ENOUGH?
When we sleep our body rests and repairs itself, processes which are vitally important to keeping healthy.
One or two nights of lost sleep might make you feel tired, but regularly being kept awake can have far more serious health consequences.
Your mental capacity begins to shrink after several sleepless nights, with brain fog, worse memory and difficulty concentrating setting in.
The immune system recharges during sleep and people who don't sleep much are more likely to get sick and run-down.
Less sleep also increases the risk of people gaining weight because it interrupts hormones which regulate hunger and appetite, leading to people eating more and not digesting it as well.
The heart also suffers when people don't sleep enough – deprivation raises blood pressure and swelling inside blood vessels, increasing the risk of heart disease or stroke.
Other negative effects of sleeplessness include links to diabetes, mental health problems, lower sex drive and infertility.
Adults should aim for about eight hours of sleep per night, on average.
This impact was stronger for girls but the researchers could not explain why.
They suggest that, in general, less consistent sleep patterns led the children to have less healthy lives.
In the paper Dr Feliciano and her colleagues wrote: 'Adults and adolescents with evening vs morning preferences tend to have shorter sleep duration, disturbed and irregular sleep, and less healthy lifestyles, including less physical activity, poor dietary habits, and greater electronic media use.'
The effect of whether people considered themselves a night owl or a morning lark, the scientists said, was 'non-significant but in the expected direction'.
Obese children are more likely to remain dangerously overweight throughout their adult lives, which puts them at a higher risk of dying young from cancer, heart disease or stroke.
The researchers and a small team of other scientists who wrote a comment to accompany the paper suggested ways of improving the teens' sleep and health.
Dr Feliciano said: 'Families should encourage consistency in their children's sleep schedules and their bed and wake times as well as improvements in their sleep hygiene by limiting electronic media and caffeine use in the evening.'
And a group of others suggested school start times should be delayed so teenagers' sleep was more consistent throughout the entire week.
The results add to past studies which have found that staying up late can have bad health consequences.
In a paper published in January, scientists at the University of Exeter studied the DNA of around 700,00 people and found those who stayed up later were genetically geared to have a 35 per cent higher chance of developing depression.
And the University of Birmingham published a study in February in which they had scanned the brains of 38 people who were either 'morning larks' or 'night owls' and found the latter had shorter attention spans and slower reactions during the day.