Fatigue

Fatigue is a derived statistic in the Hardcore mode of Fallout: New Vegas, Survival difficulty of Fallout 4, and part of regular gameplay in Fallout 76.

Fallout: New Vegas
Fatigue is a measurement of how much of a special type of damage (Fatigue damage) a character can take before they are knocked down in a state resembling unconsciousness.

If a creature or non-player character suffers enough Fatigue damage to reduce their Fatigue level to 0 or below, the value is immediately set to -30 and the non-player character will enter a state similar to unconsciousness. While in this state the non-player character cannot move or react, but is still considered alive and can suffer normal damage, up to death. Unlike unconsciousness in Fallout 3, "fatigued" or "knocked out" non-player characters cannot be looted; any attempts to interact with them will fail with the message " is Unconscious".

Fatigue is a "self-restoring" value, in the sense that it restores back to its maximum value over time. If an non-player character suffers Fatigue damage, their Fatigue value will slowly raise at a rate of one point per second until it is back at its maximum. Fatigue maximums range from 50 (low-level creatures such as mole rats) to 760 (Legate Lanius). Though fatigue values are modified by the non-player character's Endurance and Level, the value is derived from a "base" Fatigue value, set individually for each non-player character or creature. The approximate average Fatigue rating for humanoid non-player characters is 250.

Once the Fatigue value of an non-player character dips below 0 it is immediately set to -30. From this point it will increase at a rate of one point per second. Additional Fatigue damage can be applied to a "Fatigued out" non-player character, but it cannot lower the value below -30. Once the non-player characters Fatigue level climbs back to 0 they will become responsive again, and the non-player character's Fatigue level will be restored to its maximum value.

Equation

 * $$\text{Fatigue}=\text{Base Fatigue}\times(1+(\text{Endurance}\times0.2)+(\text{Level}\times0.1))$$

In plain terms, this means that for every level the non-player character's Fatigue rating is increased by 10% of the base value, and for every point of Endurance the rating is increased by 20% of the base value.

Damage sources
The number of sources which induce Fatigue damage are limited. Only five weapons inherently cause Fatigue damage: boxing gloves, boxing tape, cattle prod, Golden Gloves and Flash Bangs that were added in Lonesome Road.

All shotguns chambered for the 12 Gauge shell can also cause Fatigue damage if loaded with 12 Gauge Bean Bag ammunition.

Fallout 4 and Fallout 76
On Survival difficulty, fatigue is to Action Points as radiation is to hit points; the more fatigue one accumulates, the few Action Points they have to use for V.A.T.S. or sprinting. The amount of fatigue accumulated mostly depends on how sleep deprived one is, but hunger and thirst also play a smaller role.

In Fallout 76, fatigue works much the same way as in Fallout 4 survival difficulty. Fatigue will accumulate based on how well-fed and hydrated the player keeps their character. For gameplay purposes, sleeping is no longer required and no effect on fatigue generation.

Behind the scenes

 * When Fallout 1 was still Vault 13: A GURPS Post-Nuclear Adventure and used GURPS, Fatigue would've been on the base derived statistics along with Hit points but had to be stripped from Fallout when the GURPS license was lost. Fatigue was introduced back in Fallout: New Vegas, but functioning completely differently than how it would have worked in GURPS.