Most games assume that the characters spend eight hours of a typical day adventuring, eight hours for downtime and eight hours for resting. However, it may not be possible to rest in certain cases. In such cases, please use the following rules:

Not resting for one night requires a DC 15 Fortitude save. Success indicates that the character is resilient enough to manage through the day without sleeping. Failure results in the character gaining the fatigued state. If a character managed to rest during the night, but only for a few hours, reduce the DC of the Fortitude save by 1 for each 2 hours rested (round down; e.g. if your character rested for 3 hours, this should count as 2; 5 would count as 4 etc). This is in addition to other penalties (e.g. not regaining hit points through resting, not regaining spells etc), which are applied regardless of the result of the Fortitude save.

Failing to rest in the following nights raises the DC by 5 for each additional night of sleep deprivation. If the character is already fatigued, failing this save causes exhaustion. While exhausted, the character might need to make Will saves to avoid falling asleep during boring or repetitive tasks (e.g. watch keeping etc).