>Drinking caffeine in the morning seems like a really good proxy for whether an individual has somewhere to be that day.

Anecdotally, on a Sunday morning with 8 hours of sleep and no plans, I'm still going to caffeinate at least once, probably with breakfast. If I'm trying to stay up a long time or am sleep deprived I may caffeinate 2 or 3 times that day. I don't typically take a whole day off of caffeine based on my plans. If I have a doctor's appointment and am not supposed to eat/drink, I'll still do it later when I get back home.