Exhaustion is measured in six levels:
Level Effects (cumulative)
1 Disadvantage on ability checks
2 Speed halved
3 Disadvantage on attacks rolls and saving throws
4 Hit point maximum halved
5 Speed reduced to 0
6 Death

Exhaustion stacks; if an effect would give level 2 exhaustion to a creature suffering from level 1 exhaustion, the creature suffers the penalties of level 3 exhaustion. Certain effects will reduce exhaustion by a certain number of levels. If your exhaustion would go below level 1, you are no longer exhausted. Finishing a long rest lowers your exhaustion by 1 level, provided that you eat and drink during it (if such thing are necessary for you).

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