Fatigue (Fallout 4)

Fatigue in Fallout 4 is a derived statistic that measures the condition of fatigue damage in Survival mode.

Survival mode
In Survival mode, fatigue is like being radiated, but counts against the player's Action points (AP). If the player has fatigue, then they will have less action points that are especially needed for tactics in combat. Fatigue will increase when the Sole Survivor has not slept in a while. The amount of fatigue the player has, is displayed in red on the AP bar.