If you’re reading a book before bed, what do you reach for: a paperback, a tablet, or an e-reader? The choice you make will have an effect on your sleep, and if you answered either of the last two then you probably already know the news isn't good. Scientific evidence regarding the harmful effect of lit screens on the body’s sleep cycle has been accumulating for years, and a new study confirms it: if you want to get to sleep on time, don’t read an ebook or browse the web before bed.

The research carried out by the Harvard Medical School and published in the journal Proceedings of the National Academy of Sciences studied the sleeping patterns of 12 volunteers over a two-week period. Each individual read a book before their strict 10PM bedtime — spending five days with an iPad and five days with a paper book. The scientists found that when reading on a lit screen, volunteers took an average of 10 minutes longer to fall asleep and received 10 minutes less REM sleep. Regular blood samples showed they also had lower levels of the sleep hormone melatonin consistent with a circadian cycle delayed by one and a half hour.

The disturbed sleep associated with the use of gadgets before bedtime is blamed on the type of light emitted by electronic screens. Artificial light exposure of any kind disturbs our internal body clock (which was used to only natural light for tens of thousands of years) but the blue wavelengths emitted by fluorescent and LED lights are particularly harmful. The researchers from Harvard also found that any device with a lit screen — be it iPad, Kindle Fire, or Note — would have a similar effect. The only way to read ebooks at night and not disturb your sleep? Turn off the backlight or print it out.

Update 12/27 9:25am ET: A previous version of this article referenced the Kindle with regard to the Harvard study mentioned above. The study focused mainly on tablets, and the piece has been updated to reflect the error.