A long rest is a period of extended downtime, at least 8 hours long, during which a creature sleeps or performs 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—at least 1 hour of walking, fighting, casting spells, or similar adventuring activity—the creatures must begin the rest again to gain any benefit from it.
At the end of a long rest, a creature regains all lost HP. The creature also regains spent hit dice, up to a number of dice equal to half of the character's total number of them (minimum of one die). For example, if a creature has eight hit dice, he or she can regain four spent hit dice upon finishing a long rest. If a creature has hit dice of different amounts, they can choose which to regain.