Sleep

Syntax: sleep
With this command you can sleep whenever you want, but only if you're
sleepy enough. Sleeping helps you regenerate health, endurance points and
spell points. Sleeping with a sleeping bag will help you regenerate even
more; if you have a sleeping bag in your inventory, you will automatically
use it.  Also, while outdoors, sleeping at a campsite, and/or next to a
campfire, will enhance the regeneration effects.

Also see 'help camp'