On top of disrupting your circadian rhythm and melatonin production, blue light also directly stimulates the part of your brain that makes you feel awake, Dr. Wu said. That can be a good thing during the day, but "it's not something you want when you're trying to go to bed," he explained.
That's especially important because, even beyond blue light, using your phone or computer before bed will naturally make you feel more alert. If you're reading or watching something that gets you worked up in some way, it's going to be hard to drift off. "You get either excited or upset, and that's not conducive to sleep," Dr. Wu said. When you put together that natural stimulation with the physical effects of blue light on your brain, hormones, and circadian rhythm, you get a combination that's highly likely to disrupt your sleep.