study of lack of sleep has shown to kill brain cells, will this have an effect on schools like ranger school or normal training events?

I do agree with you but I do feel that deadly mistakes can happen with lack of sleep whether in garrison or in country IE: fratricide, falling asleep on guard, seeing things that aren't really there on patrol or guard. I also know that when in country sacrifice of sleep happens everyday and it has to happen that's just part of the job and as soldiers and NCO'S we have to walk that line with ourselves and our soldiers.

Yes I agree that it can happen but far less death will arise by having a sleepy guard than no guard at all. A sleeping rottweiler in your house will send away more criminals than an unlocked door.  If one particular guard always sleeps then yes that is a targetable vulnerability that the enemy is likely to exploit but it is still better than no one there at all.