Excessive sleepiness is the symptom of a
sleep disorder called Narcolepsy. This
sleep disorder
is characterized by excessive day time sleepiness, sleep paralysis, cataplexy where the person has episodes of loss of muscle function while awake, hypnogogic hallucinations and automatic behavior. Also, Problems with breathing (Obstructive sleep apnea), certain medications, restless leg movements and sleep walking/talking could contribute to excessive sleepiness and tiredness.