“Light is a stimulant,” says Alcibiades Rodriguez, the medical director of the Comprehensive Epilepsy Center-Sleep Center at New York University. When blue-sensitive receptors in our eyes are first exposed to light in the morning, that sends a signal to the pineal gland of our brain that shuts off the production of melatonin.
“Once you get exposed to that first light in the morning, you are supposed to fall asleep” 16 to 18 hours later, Rodriguez says. As darkness falls with night, our brains start to produce melatonin again, theoretically a couple of hours before we fall asleep.
When it’s dark outside but light indoors, it confuses this physiological system and can push back the release of melatonin, making it harder to fall and stay asleep, says Mariana Figueiro, director of the Lighting Research Center at Rensselaer Polytechnic Institute. The brighter the light, the stronger its ability to suppress the release of melatonin, which is why bright lights from phones, tablets and other LED-lit devices have a particularly detrimental effect on sleep.
An off-kilter circadian cycle not only makes it hard to get enough sleep but also increases the risk for various cancers and other diseases.