I think the humble form is only necessary for things that directly impact a superior or out-group person, usually when you are doing something for them, telling them about yourself or doing something truly outlandish like trying to interrupt them or get a word in edgewise. :sorry:
In this case, I'd probably just say "osoku natte neyou to omoimasu. Yukkuri oyasumi ni natte kudasai." It's become late and I think I'd like to go to bed. Goodnight."