I took three instructor courses over a three-day holiday weekend, and by the time it was over, I was pretty wiped-out. It's not really difficult, but just paying attention to talking people for three days can be a challenge.
One person was sent home early for mixing-up the steps in checking for unloaded/loaded status of a firearm. We approached a table with a dozen guns of different kinds, and were expected to clear them all, one guy didn't get it quite right, and he was done. So, no guarantee that you'll pass just by staying awake.