DMs need to remember that you can only benefit from one long rest per day under the RAW.
A character can't benefit from more than one long rest in a 24-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.
As a DM, there are two types of adventuring days that exist in my games.
"One and Done Days" are usually travel days, or a day with a specific incident. When they're built, I expect PCs will have full resources. This does not mean that the encounter will force them to use all those resources, but it might. Almost all of my combats have a point to them that adds to the storyline. They have challenges beyond survival. As a result, PCs can fail the combat without dying - and without it destroying the game.
"Full Days" are going to involve multiple encounters - but they often tie the encounters together so that there is no break in time between the encounters (or only a brief break - not enough for a short rest). For example, in one of my current games, the PCs have found themselves amongst a refugee fleet bound for a new land. They've been asked to investigate a ruined shoreline city. This is not the main location where the refugees will resettle, but it is located nearby, and the leaders want to understand what threat it might house. The PCs were dropped off on the shore an hour away and snuck up on the ruins ... until a very unfortunate role alerted the denizens of the 12 building ruined village to their presence. The third level party is facing kobolds. And there are
a lot of them.
The first encounter is on the outer ring of the village. Whatever direction the PCs approach from would trigger the kobolds in that zone (if the PCs were detected). The kobolds would cling to the ruin walls and attack from them while other kobolds ran to raise the alarm. The kobolds in the other sections would then come to join the fight in waves. Each wave is a separate encounter, with different tactics and different challenges - and different information to be gathered. When the fighting is on, it will make enough of a ruckus to attract the subterranean creatures that the kobolds fear (remember that they're avoiding touching the floor). When those Ankhegs appear in the town, they'll attack both kobold and PC alike.
That combat plays out as a quick combat against a few kobolds, they go down, and a little over a round later the next wave of kobolds comes in - and it is larger with a spellcaster for support. As that battle winds down, the Ankhegs will appear and attack the last of the kobolds, giving the PCs about one round to prepare (or effectively get a semi-sneak attack round on the Ankhegs). Finally, the PCs will have to contend with the Ankhegs as the remaining kobolds flee ... leaving the PCs to discover a route down under the village to the sea caves beneath it. The PCs could take a short rest before going down, but unless they are creative, they'll be faced with more threats from the caves (or brought by the fleeing kobolds) before they could complete a long rest. Once they're in the caves, there are between one and five encounters they could have, depending upon their approach, most of which can be resolved without combat. If the fleeing kobolds bring reinforcements to get the PCs, and they are down in the caves, the kobolds will not enter. This allows the PCs time inside the caves to long rest after stabilizing that environment to long rest.
That was the design. It played out differently with the PCs discovering the sea cave entrance very quickly and some of them diving into it during the first wave of kobolds. However, the basic structure still allows the PCs to break down the scenario into waves have manageable combats that roll into each other.