Sleeping in the dark is important for our health. Sleeping with the light on could be bad for us. We could get diabetes or heart disease. About 40 per cent of people sleep with a non-natural light, like a television. The worst thing is sleeping with a main light. Sleeping in the dark is more difficult for people in cities.

An expert on sleep medicine said it is healthier to turn off lights when we sleep. She studied the blood sugar levels of 20 people. Those who slept with a light on had higher sugar levels than those who slept in the dark. Light makes the brain active. This raises blood sugar. We should turn off the lights or wear an eye mask.