Twenty years ago, the discovery of a photosensitive pigment deep inside the human eye helped researchers understand how blind people can wake up naturally in the morning without seeing sunlight.

 The pigment, melanopsin, picks up the blue light of the color spectrum, which signals the brain to energize. "It was a major discovery in neuroscience," says Steven Lockley, an associate professor of medicine at Harvard Medical School. He built off this discovery to help NASA determine how astronauts in space can maintain a normal sleep schedule without a 24-hour sun cycle.

Remember science class in elementary school? When you look at a flame, the blue part is the hottest; the yellow, not as hot. That blue part of the spectrum becomes most prominent at the height—or in the heat—of the day. Think of a lightbulb as a flame: "In the daytime you want blue light, which is alerting and has you as productive as possible," Lockley says. "In the evening you want to calm the blue light, which is alerting and has you as productive as possible," Lockley says. "In the evening you want to calm the brain with lower-­temperature light and make it dimmer." This is why you should put your blue-light cellphone away when it's time for bed.