enervation — A feeling of being drained of energy or vitality; fatigue.
ennui — A feeling of listlessness and dissatisfaction arising from a lack of occupation or excitement.
exhaustion — A state of extreme physical or mental fatigue.
faintness — lacking brightness, vividness, clearness, loudness, strength, etc.: a faint light; a faint color; a faint sound.
feebleness — physically weak, as from age or sickness; frail.
heaviness — of great weight; hard to lift or carry: a heavy load.
languor — lack of energy or vitality; sluggishness.
lassitude — weariness of body or mind from strain, oppressive climate, etc.; lack of energy; listlessness; languor.
listlessness — having or showing little or no interest in anything; languid; spiritless; indifferent: a listless mood; a listless handshake.
burnout — If someone suffers burnout, they exhaust themselves at an early stage in their life or career because they have achieved too much too quickly.