ST:TNG, The Masterpiece Society, Episode 513.

First time it's been mentioned that the Enterprise might have people on different sleep schedules / time tables than the planets, ships, space stations, whatever random floating objects they've encountered.

Fun, too, because it's used as a thinly veiled excuse for why Troi woke up in regret after doing the leader of an isolationist colony of genetically engineered humans. "Oh, I'm still on Enterprise time..."