Fatigue

Spending too long without rest results in tiredness, fatigue, it negatively the creature's luck.

How it behaves
Creatures have a base fatigue value depending on their constitution, ranging from 4 at con 1, to -5 at con 24. Every 4 in-game hours (1200 seconds) fatigue is increased by 1, creatures start to get -1 luck penalty at fatigue 7 (as shown in FATIGMOD.2da). From then on, every other 1200 seconds another -1 luck penalty is applied, up to -94 luck at 100 fatigue. Fatigue can only be increased or completely reset to the base value by resting, but not decreased.

Causes

 * In-game time spent
 * Haste spell: +4 fatigue
 * Lesser Restoration, Greater Restoration spell: +7 fatigue
 * Battle Tankard makes the wearer immune to fatigue

Gameplay

 * Kicking party members from the group sets their fatigue to base, even if you invite them right back into the party

Causes

 * In-game time spent
 * Haste spell (IWD1): +7 fatigue; Haste spell (IWD2) has no fatigue increase
 * Lesser Restoration, Greater Restoration spell: +7 fatigue
 * Righteous Wrath of the Faithful applies 16 fatigue
 * Level 15 druids, any halfling or gnome with the Helm of the Trusted Defender is immune to fatigue
 * Unfailing Endurance can reduce fatigue to zero

Sources & references

 * Demivrgvs’s post on BD forum
 * semiticgod’s post on BD forum