I have the same problem, it's because of the magnet sensor - it thinks that there is a flip cover installed. Unfortunately, disabling Flipcover in settings only fixes device sleeping randomly, but it still wakes up
I also disabled flipcover, but sometimes display still turns on a every minute, sometimes just once a day.
Another question: How can I disable google now on the lockscreen? When I swift my finger from lower left corner to the center of the phone I always start voice recognition.