“Nowadays there are lots of environmental issues that play a role in altering people’s sleep patterns, and the most obvious would be the computer,” said Dr. Nancy Collop, medical director of the Johns Hopkins Sleep Disorders Center. “It emits a blue light, which is one of the most stimulating lights to the little receptors we all have in the back of our eyeballs, which send messages to the brain to say whether it’s day or night and whether we should be awake or go to sleep.”
In other words, your receptors become confused if you stare into a computer screen late at night.