Depends on the book, no?

Item:

People who read before bed using an iPad or similar "e-reader" device felt less sleepy and took longer to fall asleep than when they read a regular printed book, researchers found.

The morning after reading an e-book, people found it harder to wake up and become fully alert than after reading a regular book -- even though they got the same amount of sleep.

Story.

I'd say the writing's more important than the LEDs, but then I'm prejudiced.