This is quite a cheesy question, but let's say I have a human wizard. And I'm the last to "stand watch" so I've already slept during that rest. This is largely to avoid the incapacitated condition: see this answer to a question about concentration during long rests.
If I cast darkvision before the end of the long rest, then get all my spell slots back at the end of the long rest, do I essentially get darkvision (for the next ~8h) without expending a spell slot?
Best Answer
Yes, you can do this
That's how it goes. Spell slots are regained at the end of a long rest, and there's no rule forbidding spellcasting during a long rest.
It's effective and somewhat cheesy, but not as broken as it might sound at first. To cast that spell before ending the rest, you need to save up a slot for it. If you intend to do this constantly, you find that while you get the spell for "tomorrow" free, you have to save one of "today's" slots in order to be able to cast it just before ending the rest. During longer expeditions it won't be that much different - you still have to save the slot to use the spell, just on a different day.
Of course, it's still a convenient way to squeeze extra utility out of one's slots when they do get left unused - and a good way to start the adventure after an uneventful day during which few slots were spent.