The rest command does nothing for one turn. By default, it is mapped to the . key (dot/period/full stop). There is an option to rest when space is pressed, but this is not considered desirable.
Game time continues to run while you rest, so be wary of holding the key down. You gain no extra benefits from resting; in particular, your hit points and power do not regenerate faster.
Because rest has no special effects, you should consider sitting or searching instead of resting.
A special case is if you are burdened or worse, resting lets you wait at your unburdened speed and lets your hit points regenerate despite overtaxed status, while searching takes proportionally longer to your load. For example, this makes a difference if you are lugging a heavy sacrifice to an altar and your pet is in the way: with resting, you get an opportunity to rush in when it unblocks the way. (Arguably, you should teleport to the altar instead or kill monsters on the altar, but that is not always possible).
- Amulet of restful sleep, an item related only in name.