Lighting for telling sex of plants early, while causing least stress?

I want to use most of my females as mothers, and can't afford to waste electricity vegging males for an extended period. If any experienced growers could help answering some of my questions it would be greatly appreciated.

How soon do preflowers generally show when starting seed on a 12/12 light cycle?
Would starting on a 12/12 and gradually shifting to a 24/0 cause a lot of stress?
Also, would changing plants that sprouted from seed about 4 days ago that have been under 24/0 light to 12/12, in order to sex, then changing back to 24/0 cause a large amount of stress?
Would gradually changing the light over the course of a week relieve most of that stress?
Can stress from lighting change cause seedlings to have a greater chance of being male?

thats risky dont knw the effects probly not good. vegg them till ther big enough to top, clone on 12-12 n by the time they root they will show u there sex.

penman, dont flip to sex it will cause too much stress n possably hermie the plants depending on strain. my advice is to cut a clone from each plant asap n put in 12/12 from cut, you will see sex within 10-14 days. be sure you mark each cut so you dont mix them up. then you will know for sure n no stress on plants. reverting back to veg is a bithh imo. the cuts dont have to have roots to flower for sexing, you can just top your plants n throw the top in 12/12 if you like. hope this helps you out... ps the cut can be very small as your not really cloning just sexing then throw away, got it?

Thanks, helps a lot.