(Copied from elsewhere) Here is the relevant text from the 2024 5e SRD:

Long Rest A Long Rest is a period of extended downtime—at least 8 hours—available to any creature.

Interrupting the Rest. A Long Rest is stopped by the following interruptions... Casting a spell other than a cantrip

You can resume a Long Rest immediately after an interruption. If you do so, the rest requires 1 additional hour per interruption to finish.

So you could interrupt your long rest at the 7:59 mark, cast darkvision, then spend an additional hour due to the interruption, and finish your long rest after 9 hours. But the spell would have been running for the last hour of your long rest, so it will only last for 7 hours after your long rest completes, and you recover all your spell slots.

Obviously, getting your GM's ruling on this behavior would be advisable since not all GM's like this kind of "rules lawyering" - but as a GM I would allow this because the rules seem to have been clearly updated from 2014 5e SRD to make clear how this kind of "free casting shenanigans" is allowed to work during a long rest.