What game mechanics may be inadvertently broken by changing the time required for resting?

Question

In this question we addressed a problem in which my players were finding combat too easy, and were not having to ration their stronger abilities. After discussion I've decided to implement Dale's suggestion: redefining rest periods to 8 hours (short) and 1 week (long) in order to space the recommended number of combat encounters sensibly for a story-driven campaign while keeping my players' ability use in check.

In most instances I feel this is an optimal solution. The druid I mentioned in that question, for example, will have to be much more judicious in his use of wild shape. His spells will also regenerate at a reduced rate, so the decision of whether or not to use spell will possesses more gravity. That said, this option does come with its own problems... An example of where this breaks down in my mind:

Prepared casters, such as the druid, are typically allowed to choose spells after a long rest (~1/day), making them useful for handling utility spells (such as Shatter) that spontaneous casters would be less willing to learn due to the fact that they wouldn't use them frequently. Requiring a week of downtime to prepare new spells is far from ideal; the party won't have fun camping for a week outside a ruin to allow a caster to prep Dispel Magic in order to break a ward keeping them from entering.

Outside of things that are sensibly altered to fit a more spread-out combat schedule (e.g. wild shape use, spell slot replenishment, heals using hit dice, etc.) what game mechanics may be inadvertently broken by changing the time required for resting?

Answer 1

Almost definitely nothing.

The rest variant you have described, where a short rest is 8 hours and a long rest is 1 week, is exactly as described in in the Dungeon Master's Guide on page 267 in a section on Rest Variants.

The designers suggest this option for "gritty realism" and do not provide any warnings about what this might break or imbalance, and their silence suggests that nothing unexpected* should happen if you implement this rest variant. In other words, it should be safe to utilize this variant without anticipating the need to fix anything else and without endangering the integrity of your campaign as long as you plan for adequate opportunities to rest and avoid overloading the players with a combat pacing that presumes the standard rest schedule.

(* By "nothing unexpected," I mean that obviously your characters will be more hard-pressed with their resources, but that is an expected consequence of this variant. You shouldn't see any game-breaking side effects, however.)

Answer 2

Many things in 5e are balanced around the "adventuring day" of 6 or so encounters and 2 or so short rests between long rests. The adventuring day happens to roughly correspond to 24 hours of in-game time by default. Generally speaking, decoupling the adventuring day from the time period of 24 in-game hours has absolutely no effect on balance. Doing so is a great way to alter the in-game time pace of a campaign, allowing encounters to occur either more frequently (shorter rests) or less frequently (longer rests) than 6 encounters per 24 in-game hours and preserve the balance of the game.

Many player character abilities, including the preparation of spells, are based on occurring a certain number of times per long rest or short rest and are balanced against each other accordingly. This balance is completely independent of how long a short or long rest is. If the length of a rest is changed, balance is perfectly preserved as long as none of these abilities are changed. So, changing short rests to 8 hours and long rests to one week, but changing spell preparation to be every 24 hours would absolutely affect the balance between the different classes, as now spell preparation would be tied to short rests instead of long, making prepared spell casters far more adaptable than intended.

One more item warrants attention. Magic item recharge rates are balanced around being available so many times per adventuring day but have recharges listed as occurring every so many in-game days. This would need adjusting to match the new "adventuring day" pace in a game with different rest time. For instance, a wand that gains 1d6+4 charges each day at dawn would need to instead regain 1d6+4 charges at the end of each fortnight in a game with 8 hour short rests and one week long rests (essentially, preserving it being recharged at the end of a long rest). Not accounting for this difference would make most magical items far more or less powerful in games with longer or shorter rests.

Answer 3

To counter the problem of learning utility spells, you could impress upon them that magic users will want to use long rests, which at a week long would count as downtime, to create spell scrolls (DMG p128 seems the right place in the book). You might need to make sure that the characters come across enough spell components during their adventures to make this easy to do.

Answer 4

Fundamentally, nothing would change assuming scale of "all day" buffs was increased to match the longer "adventuring day" of several days rather than a single day. Otherwise, spells that are powered and priced assuming they last for an entire adventuring day (mage armor, long-term concentration spells like the upgraded Hunter's Mark, etc) will end up under-preforming, only lasting for a single encounter at best.

That said, this seems to be an over-correction for a simple stated problem: You find that it's too easy for the players to trivialize an adventure by taking multiple days to complete what should (generally) only take a single day. The problem you're encountering is that (because the mechanics tell them to), the players are taking multiple days to adventure, gaining a full heal between them. By making sleep only give the effect of a short rest, you keep things hard while still letting them take multiple days.

But it seems like this could be much more simply solved by encouraging them to take a single day (or at most, two or three days for a very long delve) to complete the bulk of a delve.

As such, rather than slowing everything down, it seems far simpler to just fix the pain point: the long rest. If you say that a regular long rest only allows spellcasters to change their memorized spells (not regain spell slots, or maybe only regain a very small number of slots), but leave short rests alone (or even speed them up; I've certainly noticed that as people play more 5E, the 1 hour short rest tends to turn into something closer to the 4E 10 minute rest or faster, as GMs want to keep up the pace of an adventure while still letting the PCs cut loose), you push heavily for a fast paced adventure where they burn out all their resources trying to get to the goal of the delve before time expires rather than stopping at every difficult point to do a full refresh--but where you never have to worry about them using magic items to bypass the intended change, or buff spells wearing out far too early.