Heroic though they might be, adventurers need to sleep and recover. Without magically means they are forced to do what the rest of us do, sleep and take it easy for a bit.
A short rest is a period of downtime, at least 1 hour long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds.
A character can spend one or more Hit Dice at the end of a short rest, up to the character’s maximum number of Hit Dice, which is equal to the character’s level. For each Hit Die spent in this way, the player rolls the die and adds the character’s Constitution modifier to it. The character regains Hit Points equal to the total. The player can decide to spend an additional Hit Die after each roll. Hit Die are recovered after a long rest.
A long rest is a period of extended downtime, 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.
At the end of a long rest, a character regains all lost Hit Points. The character 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). So two long rests are required to regain all of the characters Hit Dice.
A character can’t benefit from more than one long rest in a 24-hour period (you try and take two long sleeps in the same day), and a character must have at least 1 hit point at the start of the rest to gain its benefits. So if the character is at zero hit points, they have to get to one before they can gain the benefits of a short or long rest, which happens after 1d4 hours of resting at zero hit points. Those pesky near-death experiences really take their toll on your characters ability to bounce back.
Of course, this is a fantasy setting, and magical healing can speed up much of the recovery process but that’s covered in the spell or item’s description. Typically, hit points are easy to recover, hit dice, not so much.