It's interesting to see Draw Steel!, and likely other RPGs, address this issue in their core mechanics.
In DS!, your most powerful spells/abilities require hero resources, which you don't have at the start of your first fight of the day. You build them up each round so that by round 3 (combat generally lasts 3 to 5 rounds), you can hit with your hardest spells / combat abilities.
More importantly, you get a Victory for each combat or obstacle you overcome. At the start of the next combat, you have an extra hero resource for each Victory you start the combat with. So you hit harder, earlier. But you're also likely dinged up and closer to needing a rest.
When you take a long rest, you get healed back up and gain experience points based on the number of Victories you have. But then you lose those Victories and start the next cycle back at zero. Also, you can't take a long rest in a standard dungeon, you need to retreat back to a completely safe location.
So from a design perspective, if you put a boss fight at the end of a chain of encounters, you have a pretty good sense of how powerful the heroes will be when they tackle the boss. Either they'll be sporting a bunch of Victories to hit hard from the get go, or they'll be fully rested up and have the endurance to go an extra round or two. But they won't have both, like they would in D&D after a rest.