Scientists at the Institute of Cancer Research, while studying over 100,000 British women, have discovered a link between obesity and the amount of light in their bedrooms at night. They found that the more that the women were able to see across their bedrooms at night, the larger their waistlines.
Light in the bedroom at night disrupts our 24-hour body clock (known as our circadian rhythm), decreasing the production of melatonin, which regulates sleep patterns and increasing the production of the stress hormone cortisol, which can result in over-eating. Therefore, people who sleep in more brightly-lit rooms have a higher tendency to become obese than those whose bedrooms can be darkened more effectively at night or who wear an eye mask at night.
For more information, see http://www.bbc.co.uk/news/health-27617615