Staying up late could be associated with weight gain in teenagers, according to a study. Researchers in the US found that each additional hour adolescents stayed up during the school week has been associated with an increase of 2.1 points in body mass index.

The authors of the paper, published in the Sleep journal on Friday, suggest that the results – observed in a sample of more than 3,000 participants – highlight that bedtimes could have a role to play in weight management.

Although BMI (body mass index) does not distinguish between lean muscle and fat, and the researchers from the University of California, Berkeley, and Columbia University did not measure waist circumference, the study will raise further concerns about the reported relationship between lack of sleep and obesity.

The authors said they were surprised to find that the relationship between bedtime and BMI – calculated using an individual’s height and weight – was not significantly changed or moderated by total sleep time, exercise frequency or screen time.

However, the relationship between bedtime and BMI was significantly reduced with the inclusion of fast food consumption. The paper said this made the case for future research on the effect of later bedtimes upon dietary habits.

Lead author Lauren Asarnow, a doctoral candidate at the University of California, Berkeley, said: “The results are important because they highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management concurrently and in the transition to adulthood.”

The study incorporated three waves of data from the National Longitudinal Study of Adolescent Health between 1994 and 2009, with the youngest participants aged 12 and the eldest 32.

Sleep and circadian variables were determined via self-reported measures at all three waves and investigators measured height and weight at each stage.

The authors believe their research to be the first to investigate the relationship between bedtime and BMI longitudinally in an observational study.

A study by the University of Bristol, published in March, found that if you need a lie-in at weekends to make up for lack of sleep in the week, you may be at risk of obesity and type 2 diabetes.

As well as lack of sleep being linked to obesity, night shift work has also been linked to being severely overweight, because sleeping during the day burns fewer calories than at night.

Experts suggest that a mismatch between the body’s internal circadian clock and sleep patterns can disrupt normal metabolism.