Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 39 to 43.

Everyone knows that good sleep is vitally important for our health. Not everyone knows how important it is to sleep in the dark. A new report says that sleeping with the light on could be bad for our health. Researchers say that turning off the light when we sleep helps to keep away diabetes and heart diseases. Sleeping with thelight on could increase the risk of getting these diseases. The researchers said that around 40 percent of people sleep with some sort of artificial light. They said even the light from a television or alarm clock could affect our health. They added that the worst thing to sleep with is a main light. Sleeping in the dark is more difficult for people who live in cities where there is a lot of outdoor light at night.

The researchers are from the Northwestern University Feinberg School of Medicine in Chicago, USA. Dr Phyllis Zee, who is an expert in sleep medicine, is the lead researcher. She said that it is healthier for us to turn off all lights when we sleep. Her team 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 blood sugar levels the next morning compared to those who slept in total darkness. Dr. Zee said this is because light stimulates brain activity, which raises blood sugar levels. She said there are three things we can do to reduce the risk of illness. The first thing is we should turn off the lights. Moreover, we should never sleep with white or blue light. Finally, it is a good idea to use a blackout curtain or wear an eye mask.

The pronoun "They" in paragraph 1 refers to ______________