Term: Rest, Long

A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch. If the rest is interrupted by a period of strenuous activity—at least 10 minutes of walking, fighting, casting spells, or similar adventuring activity—the characters must begin the rest again to gain any benefit from it.

At the end of a long rest, a character regains hit points equal to their remaining Hit Dice maxed out, plus other modifiers (health and CON per level, Ancestry Healthy, etc), and then regains half their Max Hit dice (recovering lower/small hit dice first). The character also regains spent Hit Dice.

For example, if a human player character (8+1) with 2 CON and six d8 Hit Dice Max, and spent 4 over the course of the Adventuring Day; they regain 2d8 HD's worth (or 15(Ancestry) + 12(CON) +16(HD), or 43) of HP, and then regain 3 spent Hit Dice upon finishing a long rest, coming off the Long Rest with 5 HD.

A character can take more than one Long Rest an Adventuring Day (up to 2), but can only benefit/recover from the first within a 12 hour period. A character must have at least 1 hit point at the start of the rest to gain its benefits.