Previous research has linked too much light at night to depression

Is thought that light exposure may affect metabolism and therefore weight

A poor set of curtains may not just mean you wake up too early - they could also make you fat.

A study has found that sleeping with too much light in the room increases the risk of obesity in women.

Greater exposure to light at night raised both Body Mass Index (BMI) and waist size in more than 113,000 women taking part in the British study.

Danger: Being exposed to light at night may increase the risk of obesity by disrupting the body clock

The Breakthrough Generations Study followed the women for 40 years in an attempt to identify root causes of breast cancer. Obesity is a known risk factor for the disease.

Professor Anthony Swerdlow, from The Institute of Cancer Research in London, said: 'Metabolism is affected by cyclical rhythms within the body that relate to sleeping, waking and light exposure.

'The associations we saw in our study between light exposure at night and obesity are very intriguing.

'We cannot yet tell at this stage what the reason for the associations is, but the results open up an interesting direction for research.'

Co-author Dr Emily McFadden, a visiting researcher at the the institute, said: 'Because all the information was collected at the same time, we cannot tell the sequence of events.

Negative: Other research has found the disruptive effect of nighttime lighting on the body may contribute to depression and mood disorders

'But the associations we found are consistent with previous research examining light exposure and metabolism, and further investigation is needed.'

The study was funded by the charity Breakthrough Breast Cancer, whose senior research officer Dr Matthew Lam said: 'These findings add weight to previous results from animal studies that looked into how light exposure, circadian rhythms and metabolism could all be connected in some way.

'It's too early to suggest that sleeping in the dark will help prevent obesity, a known risk factor for breast cancer, but the association is certainly interesting.'

The findings are reported in the American Journal Of Epidemiology.

Previous research has drawn similar conclusions. A team at Ohio State University examined how nocturnal light affects weight, body fat and glucose intolerance (the underlying cause of late-onset diabetes) in mice.

They found that persistent exposure to even a little night-time light caused increases in all three.