If someone you hired to be a night shift aide keeps falling asleep, that's not a good sign. I'd start watching other aspects of his or her behavior more closely.
If the person is otherwise a good caregiver and has other good qualities, bring the issue up and discuss it. Before taking action, try to find out what's going on -- is he or she not getting enough sleep during off-hours? Working a second job? If there's a problem, be clear that the caregiver needs to address it.
Night aides need to get enough sleep at other times so they can present for the older adult during the hours they're paid for. Be firm about the fact that this is a safety issue. If an aide's sleeping on shift, he or she is not going to be aware and able to handle an emergency.