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?