Computer enters sleep mode at the beginning of games

imac retina 27 osx 10.10.5

at the beginning of most games as soon as the champions load onto the map my computer will enter into a sleep mode, i have to move my mouse around and hit keys until it fully enters the mode and then it'll load back up when i wake the computer up. No problems from there on. Anyone have any advice? Only happens in league, all sleep mode settings set to never. Confused. just started happening past couple months been playing for years.

thanks