>I suppose something that engages you for hours could appear unscheduled while sailing
I would say, generally speaking, that comprises the bulk of the time. Most likely you will spend more time in unscheduled multi-hour long tasks than anything else over the course of an extended trip.
Well I guess that shows how much I know about sailing (I've done it once or twice, but never in a hurry or with a lot of distance between ports). Which of these cannot be delayed 45 minutes in order to accommodate a nap?
Wind shifts, something breaks, something blows overboard, random water where it shouldn't be. Something is making a funny noise. Some alert on the radio that you didnt catch all of it. Odd smell. People speaking excitedly in a foreign language about something. Something breaks again, hooked a fish, approaching vessel with no signal, coast guard inspection, unexpected military craft telling you to change course. Sure there is downtime, but it's lucky to have it remotely scheduled.