In Dungeons & Dragons 5th edition, there are two kinds of rests that your players can take. There is the "short" rest and the "long" rest. Each one is defined not only by the its mechanics but how it integrates with the role playing and story being told at the table. First, let's break down how the rules define both of these aspects of the long and short rest.
Mechanically, a short rest must last 1 hour at the end of which they may spend dice from their remaining Hit Die pool to recover 1 HD + Con Bonus in Hit Points. No HP is recovered "automatically" in this process, Hit Die must be spent. They can be taken 1/hour, so basically as often as the players and DM wants. In terms of role play, the character can do nothing "more strenuous" than eating, drinking, reading, or tending to the wounds of themselves or allies.
On the mechanical side of the long rest must last 8 hours in total length, at the end of which the character recovers 100% of their HP and 1/2 of their total Hit Die pool. In addition, most spell casters will recover all of their spell slots and be able to prepare a new list of spells. For the narrative of a long rest the characters must sleep for 6 out of the 8 hours (unless they're an elf) and during the 2 hours of wakefulness may only perform "light activity": eat, read, talk, and stand watch. If they do anything the PHB refers to as "strenuous activity" (1 hour of walking, fighting, casting a spell, or "similar adventuring activity"), the long rest must be started over in order to gain any of its mechanical benefits.
The short rest as it stands works fine for my games. Hit Die are a very finite resource as are most of the class abilities that recover during a short rest. My problem stems from the long rest. Mechanically, a long rest will takes a character in nearly any condition and bring them to perfect fighting condition. At worst, they will be missing 1/2 of their Hit Die. The reasoning for this is described in the DMG on page 267 where it states: "The rules for short and long rests presented in chapter 8 of the Player's Handbook work well for a heroic-style campaign. Characters can go toe-to-toe with deadly foes, take damage to within an inch of their lives, yet still be ready to fight again the next day."
To their credit, they knew that this would feel a little too easy for some DMs and players so they provide a "Slow Natural Healing" and "Gritty Realism" variant on the resting rules. With the Slow Natural Healing rule, no HP is recovered after a long rest - it may only be recovered by spending HD. Gritty Realism tweaks the time each rest takes. A short rest becomes 8 hours while a long rest becomes 7 days. Seven days! They acknowledge "this puts the breaks on the campaign" and recommend it only for campaigns where combat is rare or to be avoided.
Neither of these address what really grinds my gears about the long rest rules. I dislike how incredibly powerful it is at healing characters in any environment but I don't want to introduce rules that simply make healing take forever. My players and I still want a game with a fair amount of combat. These variants and the standard short/long rests leave out an important factor that if taken into account solves my problems. That factor is the quality of a long rest, not simply whether or not you have a place to lay down for 8 hours.
Sleeping in a dungeon with roaming monsters on a hard stone floor with one eye open looking for danger would be significantly less restful than sleeping in a warm bed in the heart of Waterdeep surrounded by guards. The way I perceive it, the long rest described in the PHB represents near ideal resting conditions. A warm fire, low to no chance of being attacked in the night, and at the very least a bedroll to sleep on. What about when they try to sleep in the heart of Neverwinter Wood with the sounds of hungry predators prowling about the camp?
I wanted rest mechanics that more closely represented the role play aspects of trying to sleep in some forsaken tomb. In the style of D&D I prefer to run I don't think you should wake up feeling incredibly refreshed in a dungeon. It has to be difficult trying to catch some Zs on a slab of ancient granite hundreds of miles from the nearest inn knowing there are liches or worse going bump in the night. I could force the issue by simply throwing monster encounters at them every time they try to sleep in a dangerous area or restrict them to short rest but those solutions feel cheap and "gamey" in an unappealing way to me. It also fails to really address the problem of 100% health restoration after a long rest in the dungeon that makes the game a bit too easy for my taste.
Hence, the "medium" rest.
The medium rest lasts 4 hours, 1 of which may be spent on light activity. At the end of the medium rest, a character regains 1/4 of their HD (minimum 1). In addition, they recover their level in HP and may choose to spend HD in the same manner as a short rest. Only two medium rests may be taken in a 24 hour period, they may be taken back to back. Class abilities which may be used on a short rest may be used during only one of the two medium rest periods.
This represents the characters stopping to quickly patch themselves up and catch an hour or two of shuteye as best they can. With these rules taking two medium rests is always worse than taking a long rest and it will gradually wear the party down creating a more dangerous tension the longer they stay in a dungeon. Eventually, they will have to find a safe haven or create a very secure area through magical means or otherwise in order to get a long rest. My hope is this will encourage some creative thinking from the players as they look for ways to create a safe haven for themselves.
I'll be testing this out on my players in my ongoing Princes of the Apocalypse campaign and I will report back in after a few sessions and we'll see if it's working at all as I hope it does.