Ahglock said:
On the other hand if a dozen orcs ambush the party and they barely live through the nighttime ambush, I'd think going back to sleep and getting a solid rest would be fairly hard.
I would have thought that sleeping after that would, assuming they're confident it won't happen again, be easier.
Now whether they gain the full benefits is another question.
I think the real question being asked here is "should a nighttime encounter reduce the resources available the next day?"
I think the followup question is "is it fun to have a nighttime encounter reduce the resources available the next day?"
Personally I think the answer is generally no.
Specifically if the players are taking reasonable caution when setting up camp, I'd at most say that the ambush means their rest time is extended by the length of the engagement. The purpose of the encounter was to push the adventurers that little bit more at the end of a day, not to make their next day harder. If you try to make them continue adventuring without getting the full benefits of rest, they'll just take a day off and rest again.
If the aim is to punish the players for just not bothering (we sleep in the guard's bunks! we sleep in the dungeon's kitchen etc), or move them on from somewhere (we sleep at the base of the city monument to save on inn fees!) then increase the amount each interruption adds to the time needed. The second adds an hour, the third 2 hours etc. with a hard max of 6 hours, or the DM giving up the campaign as a bad idea.
Oh, and given the stated purpose of a nighttime encounter - no, the party gets no benefit from partial rest.