I read nocturnal as meaning 'at night' or during 'the night'. We know that dialysis won't be needed every night. A working person might be pleased to be able to get some medical procedure done at night instead of having to fit it in to the day.
This is one of those situations where no entirely correct word comes up, so you use the one with the least problems. "Nocturnal" is a bit poetic, but is right. "Nightly" suggests every night. I suppose you could say "nighttime".