These are not fully specified yet - we still have a lot of work to do.
Each level of Exhaustion adds 1 to your Action Load. This means that at the beginning of each of your turns, instead of resetting your Action Load to zero minus your Action Point Bonus you would set it to your current Exhaustion Level minus your Action Point Bonus. This logic also applies to Aspect Checks outside of combat, including Initiative.
Levels of Exhaustion can be gained in various ways such as not resting enough, because of a disease or other affliction, or by abilities where you can abuse your power.
(temporarily reduces Evasion)
(temporarily restricts movement)