A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps or performs only light activity: reading, talking, eating, or standing watch for no more than 2 hours. If the rest is interrupted by a period of strenuous activity—fighting, casting spells, or similar adventuring activity or at least 1 hour of walking—the characters must begin the long rest again to gain any benefit from it.
At the end of a long rest, a character regains 2 hit points per character level, up to his maximum hit points, and 1 point of ability damage to each damaged ability score. In addition, any ability measured in uses per day is restored to its maximum number of uses.
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.