noun

a feeling of being sleepy and lethargic; sleepiness.

this drug can cause drowsiness

sleepiness,somnolence,tiredness,fatigue,weariness,exhaustion,sluggishness,lethargy,listlessness,torpor,enervation,lifelessness,laziness,indolence,inertia,lassitude,apathy,debility,doziness,dopiness,grogginess