A Long Rest is a period of extended downtime, at least 8 hours long, during which a character sleeps or performs light activity for no more than 2 hours.
At the end of a Long Rest, a character regains all lost Hit Points and up to half of their total number of Hit Dice.
A character can’t benefit from more than one Long Rest in a 24-hour period, and needs at least 1 Hit Point at the start of the rest to gain its benefits.