Sleeping with the TV or a bedroom light on makes one in three women more likely to become obese, a study has found.
Researchers tracked 44,000 women in the first ever population study to establish the link between artificial light at night and piling on the pounds.
Having any kind of artificial light in the bedroom meant women were a fifth more likely to be obese five years later.
Women who regularly fell asleep watching television or with a light on were found to be most at risk.
They were 17 per cent more likely to gain 5kg over the study period and 13 per cent more likely to see their body mass index (BMI) increase by 10 per cent.
Their BMI was 22 per cent more likely to be in the “overweight” category and 33 per cent more likely to reach “obese” levels after five years.
Women from all over the US aged 35 to 74 were studied by researchers at Triangle Park National Institute of Health in North Carolina.
Author Dr Yong-Moon Mark Park said: “Exposure to artificial light at night (ALAN) may contribute to the obesity pandemic.
“ALAN has been shown to suppress expression of circadian clock genes and, in turn, alter feeding behaviours and lead to weight gain in rodents. There has been little investigation of the association between ALAN and obesity in humans.”
Forms of ALAN included from clock radios and small night lights, as well as from other rooms or shining in from street and car lights outside.
Professor Malcolm von Schantz, of Surrey University, added: “These findings won’t change the advice to maintain sleep hygiene, and avoid light and electronic distractions in the bedroom, but they add further strength to the case.”
