Overcast

Effect
Exhaustion reduces the character's maximum energy. When Exhaustion occurs, the caster loses 10 maximum energy. The amount of maximum energy lost is shown at the right end of the energy bar as greyed out. Maximum energy is recovered at a rate of 1 point of energy every 3 seconds.

Exhaustion stacks. A character repeatedly casting exhaustion spells will repeatedly reduce their maximum energy, lower and lower each time. Exhaustion can lower maximum energy below zero. In this case, the deficit is regained at 1 point per 3 seconds before regaining positive maximum energy.

Source
Exhausting yourself Some Elementalist spells exhaust the caster. Exhaustion triggers at the start of casting (like adrenaline or energy costs), not upon resolution, so the caster is exhausted even if the spell is cancelled or interrupted.

The following Elementalist spells cause exhaustion:
 * Energy Storage: Ether Prodigy
 * Air Magic: Chain Lightning, Lightning Surge, Gale, Mind Shock, Shock
 * Earth Magic: Earthquake, Obsidian Flame
 * Fire Magic: Meteor Shower, Mind Burn, Meteor, Double Dragon
 * Water Magic: Maelstrom, Mind Freeze

Exhausting enemies:
 * If Exhausting Assault successfully interrupts a spell, the victim gains Exhaustion.
 * Arcane Languor causes exhaustion when the victim of this hex casts a spell.
 * The monster skill Crystal Haze causes exhaustion when the hexed character uses any skill that costs energy.

Removal
Changing zones removes exhaustion. The only other method is the natural regeneration of one maximum energy every three seconds.

Prevention
Glyph of Energy prevents your next spell cast from causing exhaustion.