What punishment would you expect if you fell asleep on sentry duty?

It makes no sense to punish a soldier that was caught sleeping while on guard duty. Competent squad and platoon leaders should always know how exhausted their soldiers are and adjust their plans and rosters accordingly:

The more tired your soldiers are, the shorter the time they should spend on guard duty. It might happen that you have to change your guards every twenty minutes, but so be it. It’s a pain in the but better than having people falling asleep.

If possible, send two soldiers to each position. This way, they can watch each other and the possibility that both fall asleep is much lower than if you only send a single guy

there really is life in the deepest part