The problems people usually have with light is seen with pin holes of continuous light. I have from day one observed my plants at night and have no problems flashing light on them if there is something to observe during the night. That doesn't bother the plants IMO, but how long you can shine your torch for I dont know. I'm not sure what the cut off in time is what I mean to say but it's surely longer then you or I would care to sit there. I do know that if it's not completely pitch black when you close the door (after observations), then light problems will arise which reveals itself via hermies and algae. The problems are found either with continuous pin light or short cycles in timers. Have a brown out which trips your timer for instance, then not bringing the lights back on the next day as they should, that sort of thing causes problems, not flashlight observations during night cycles.