A long rest is a period of extended downtown, at least 8 hours long, during which a character 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 characters must begin the rest again to gain any benefit from it.
. . . [snip] list of benefits from completing a long rest [/snip] . . .
"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."