This has been happening to me for around a month. Screen turns on by itself and is on the lock screen, and it stays on for however long I have my 'screen timeout' set for, then turns off again. I downloaded an app called 'Spare Parts' and it has a function that tells you what apps or phone functions turn on during the day and keeps count to how many times they are used. I downloaded this because of a recommendation from another thread to try and see if any apps are running in the background, and how many times they activate. When I went to bed, everything was on zero. When I woke up in the morning my battery was down from a full charge to only 50%! When I checked 'Spare Parts', it showed that my screen had turned on 31 times that night and that NO OTHER APP OR FUNCTION HAD ACTIVATED WHATSOEVER. I had my phone on automatic brightness and screen time out at 5 minutes (I read a lot of web pages and hate it when the screen turns off because I haven't touched it). The only conclusion that I can come to is that there is something wrong with the phone. I need to go and swap it out and see if that does anything.
In the mean time I have set a smart action that is set for: When it is night time, at my house, and there is no phone movement... set brightness to the lowest setting, and screen time out at 15 seconds. This way my battery doesn't drain so much while I am asleep.