DnD 5e - Resting and Healing
Resting is an important part of adventuring. You can't spend every second of every day marching and fighting. To recharge abilities and spells, to recover hit points with natural healing, and to handle certain tasks like identifying magic items, you need to spend some time resting. This takes two forms: short rests and long rests.
Rules for resting applies to creatures of all types, including constructs like golems and undead like zombies and vampires. Unless their stats specify otherwise, all creatures can rest and regain hit points and other features like spell slots which are recovered by resting.
A short rest is a period of downtime, at least 1 hour long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds. If the rest is interrupted, such as by combat, you must start over.
At the end of a short rest, you may spend one or more hit dice to heal, provided that you have remaining hit dice. If you do so, you roll the hit die, add your Constitution modifier, and regain the resulting number of hit points.
For example: Joe is a level 2 fighter with 14 Constitution. After taking some damage in a fight, Joe and his party decide to take a short rest. At the end of the short rest, Joe decides to spend both of his hit dice (d10's because Joe is a Fighter). and recovers 2d10+4 hit points (2 d10 dice, plus his Constitution modifier added to each die rolled). Later in the day, the party takes another short rest, but Joe has already spent all of his hit dice so he's unable to use hit dice to recover additional hit points.
If you have hit dice of multiple sizes (common for multiclassed characters), you may decid which hit dice to roll, if any.
For example: Greg is a Rogue 2/Barbarian 1, giving two d8 hit dice and one d12 hit die. When Greg takes a short rest, he may roll any combination of hit two d8 hit dice and his one d12 hit die, adding his Constitution modifier to each die rolled as normal.
For more on hit dice, see Hit Dice, later in this guide.
A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps or performs light activity: reading, talking, eating, or standing watch for no more than 2 hours. If the rest is interrupted by period of strenuous activity - at least 1 hour of walking, fighting, casting spells, or similar adventuring activity - the characters must begin the rest again to gain any benefit from it.
The allowance for standing watch and for one hour of interruption are important. While resting in a potentially dangerous area, such as an untamed forest or a dungeon, it's important to set a rotating watch to protect the rest of the party while they sleep. The 1-hour interruption allows the party to endure an overnight combat without forcing them to restart their rest, which could lead to another rest interrupted, forcing the part into a horrifying vicious cycle where they never quite manage to complete their rest.
At the end of a long rest, a character regains all lost hit points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character’s total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a long rest.
The rate at which creatures recover hit dice has some interesting implications. If you're at an odd level, it can take up to three days to fully recover your hit dice because of the "round down" rule. For example: a 3rd-level character recovers just one hit die per day, and a 5th-level recovers two, so both will be short one hit die after expending all of their hit dice then completing two long rests. This also means that, even though you're back to full hit points it can be risky to line up full days of fighting back-to-back, especially if you don't have other sources of healing (magic, most likely) readily available.
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. Short rests and long rests are mutually exclusive; you may not take a short rest while taking a long rest.