Nytmare
David Jose
That will seriously throw off 5e. Challenge & balance is 5e is realized only over a long series of 6-8 relatively minor ("moderate-to-hard" is relative, perhaps misleading) encounters creating an attrition-based challenge overcome via resource-management and class diversity among the PCs.
Shorter days were going to be necessary then, anyway. Not 1-2 encounters, but shorter. You might consider just taking the long rest mechanic off the table, entirely, but allowing a short rest after every encounter. Class balance will still be badly distorted, but hp management might work out OK.
I think that changing the scale of the short and long rests will end up doing what you're suggesting.
The problem I was running into was that the story being told was about the group strategically tracking down and hunting groups of bad guys with days of searching, travel, and foraging for food and water in between fights. The search and destroy aspect was going to be just as much a part of the "game" as the combats, but with the players getting a full HP and special powers reset between almost every single fight, I had no idea how to scale the encounters.