Being addicted to my iPad, I took a closer look at the study.
It showed there was no effect after one hour. The effect did appear after two hours, and how strong it was depended on how far the tablet was from the eyes, and the light level of the app.
Smart phone screens and computer screens can have the same effect. The study’s authors wrote, “We recommend dimming these devices at night as much as possible in order to minimize melatonin suppression, and limiting the amount of time spent using these devices prior to bedtime.”
It makes a good case for what many of us used to to—reading a good (printed) book before going to sleep.