Apple iPad Air 2-14

We all know it's bad to spend too much time staring at screens, but a new study from the Harvard Medical School reveals just how dangerous it can be. It turns out that reading from a tablet or any light-emitting display before bed can wreak havoc on your sleep cycle and worse.

The study involved locking 12 test subjects in a lab for two weeks. For the first five days they read paper books, but were given an iPad for five days after that. At the end, blood tests revealed their melatonin levels (a sleep hormone) had dropped after going to bed with a screen.

"The light emitted by most e-readers is shining directly into the eyes of the reader, whereas from a printed book or the original Kindle, the reader is only exposed to reflected light from the pages of the book," said research lead Professor Charles Czeisler in an interview with the BBC. He added that beyond depriving you of a good night's sleep, the long-term effects could include cardiovascular disease, obesity and even cancer.

The issue boils down to your internal clock, which uses natural light to tell the time of day. Too much artificial light confuses your body and keeps you awake. Fixing the problem may mean cutting back on your screen time, picking up a book instead, or just disabling your device's backlight late at night. If you absolutely must check e-mail before you fall asleep, you might want to consider third-party apps such as F.lux, which works on Mac, Linux, iPhone, iPad and Windows, or other apps such as Twilight for Android, which forces the display to block more blue light, which apparently also affects sleep.