Sleep is important for our health. Not everyone knows it is important to sleep in the dark. Researchers say sleeping with the light on could be bad for us. Turning off the light when we sleep helps to keep away diabetes and heart disease. Light could increase the risk of getting these diseases. Around 40 per cent of people sleep with some sort of artificial light, from a television, alarm clock or something else. The worst thing is sleeping with a main light. Sleeping in the dark is more difficult for people in cities, where there is a lot of outdoor light at night.

The researchers are from a university of medicine in Chicago, USA. Researcher Dr Phyllis Zee is an expert in sleep medicine. She said it is healthier to turn off all lights when we sleep. She conducted a study of the blood sugar levels of 20 people after nights of sleep. The people who slept with a light on had higher sugar levels than those who slept in darkness. Dr Zee said light stimulates brain activity, which raises blood sugar. She said there are three things we can do to reduce the risk of illness: turn off the lights, never sleep with white or blue light; and wear an eye mask.