I think I might shower late at night next to a sensitive sleeper's room
Why would you want to do that?
I want to see if I can wake him playing guitar in the shower.
It is wrong to disturb other people on purpose when they are sleeping. You can go somewhere else to practice if you want to play guitar.
That's too much work for little fun. I didn't say anything about annoying the guy, just playing guitar.
It would be annoying to him if you woke him up. You should try and be more mindful of people trying to sleep and not make any noise. They might have obligations they need to get up for in the morning.