When you curl up in bed, consider reading an old-fashioned printed book rather than a smartphone or tablet.

Your sleep should be deeper and more restful.

That’s the finding of a study in the Proceedings of the National Academy of Sciences.

Researchers had 12 healthy young adults read either a printed book or an iPad for four hours before bed during five consecutive evenings.

During the fifth night blood samples were collected every hour via an IV during both the reading and sleeping periods.

The research team assessed sleep time and REM cycles, and the subjects self-rated their sleepiness every evening and morning.

All participants read for five nights on the iPad and for five from a book.

As anticipated, reading print made for better Zzzs.

Participants reading the iPad took about 10 minutes longer to fall asleep, secreted less sleep-inducing melatonin, and shifted their internal circadian clock.

They also reported feeling more tired the next morning.

The study’s first author, Anne-Marie Chang, then of Boston’s Brigham and Women’s Hospital, says that devices like the original Kindle, that do not produce their own light, should likely still be a good choice for bedtime reading.

But when it comes to those backlit gadgets, probably best to read those over your morning coffee.