"A weary mind on the battle field can be your true worst enemy. Ask the men who'd wanted five more minutes of sleep. it could have damn well saved their lives." - Baltur Cross, Leader of the Phantom Cross.
Focus is one of the Survivial Stats of the avatar. Focus will drain overtime, accelerated by crafting and spell casting. Sleep can be achieved by resting the character on a bedroll or bed. This will fully restore focus. Field Flowers can help to regain Focus when other means are not available.
Once focus is drained, it will slowly drop your health points, potentially leading to death if left at 0 for too long.