Resting allows a character to regain lost hit points and replenish spell slots. Any creatures summoned by that character will be unsummoned, and any lingering spell effects cast by that character will dissipate upon the beginning of a rest. A completed rest cycle also removes magical and extraordinary effects on the rested character. Supernatural effects, including most poisons and diseases (but not necessarily the damage caused by these) are not removed by resting. In addition, resting will cause a disease to progress, forcing the resting character to make a saving throw to attempt to resist the disease.
Resting can only be started in areas that permit resting, and only if there are no hostile creatures within a certain distance (even if there is no access from the creature to the character attempting to rest). Some modules (including the Shadows of Undrentide campaign) will allow resting in some areas only if a character is in a certain part of the area, possibly requiring that all doors to the room be closed first.
Resting can be interrupted (by player action or monster attack). If this occurs, only partial benefits of rest are obtained. Hit points are restored gradually (linearly) over the rest period, while spell slots are restored at irregular intervals, starting with level 0. The slots for the highest spell level a character can cast are only restored if the rest cycle completes. Some modules (including the Hordes of the Underdark campaign) provide the possibility of rest being interrupted by "wandering" (technically spawned) monsters.
Resting restrictions are somewhat common in some modules, particularly persistent worlds. These restrictions often include a limit on how often a character can rest, and sometimes a limit on how many hit points are restored. (Technically, full hit points are restored, followed by the character being damaged to remove the points in excess of what the world allows per rest cycle.) In addition, special items, such as rations, firewood, or a bedroll, may be required in order to rest.