Fatigue

[edit] On Oblivion

Fatigue is how tired you are. Low Fatigue hurts your effectiveness, especially when doing melee attacks. Fighting hand-to-hand, swinging a weapon, or jumping may reduce your Fatigue. If your Fatigue falls below zero due to a spell or other special effects, you are knocked down. Walking, resting, waiting, potions, and spells can restore your Fatigue. Your maximum Fatigue is the combined values of your Strength, Endurance, Agility, and Willpower.

[edit] On Morrowind

Fatigue is how tired you are. Low Fatigue makes it harder to successfully perform actions. Running, jumping, swimming fast, and swinging a weapon reduce your Fatigue. Walking, resting, waiting, potions, or spells can restore your Fatigue. Your maximum Fatigue is the combined values of your Strength, Endurance, Agility, and Willpower.