Errata

In OSR+, heroes recover 1 HP and all their MP after a full night's rest, but the rules are silent regarding what happens when heroes rest for part of the night.

The logical answer is that you recover some fraction of your HP and MP based on how much you sleep, but having any sort of fractional rules in OSR+ runs afoul of the spirit of the game, which values simplicity and on the spot rulings when there are gaps in the rules.

A full night’s rest means uninterrupted sleep for 8 hours. If you’re awakened for more than an hour in the middle of resting, you engage in strenuous activity for any length of time after being awakened, or you sleep for less than 8 hours, then you do not regain any HP from resting. The same is true of MP recovery. There is no concept of a “short rest” in OSR+, so splitting up your rest throughout a day or night cannot benefit you, although the GM may permit you to regain HP or MP after an interrupted rest at his discretion.

Hit Points