Norwegian researchers have found people who read from an iPad for 30 minutes prior to going to sleep felt less sleepy than when reading from a book for the same period of time. EEG data demonstrated delayed and reduced slow wave activity (deep sleep) in the brain after sleep onset compared to when participants read from a book. This is probably due to how blue light signals the brain it is daytime.
PositiveTip: For better sleep use real books, or make sure you have updated your iOS device so you can use the new nighttime mode.