The main pathologies causing drowsiness or hypersomnia are sleep-related. The various types of sleep apnea are one of the main causes of hypersomnia and drowsiness but some causes are neurological, such as the restless leg syndrome that makes falling asleep difficult or disrupts sleep.
Some rare but very important diseases such as narcolepsy, idiopathic hypersomnia, Kleine-Levin syndrome are also sleep-related disorders. Let’s also not forget neurological diseases such as neuro-degenerative disorders – Parkinson’s for example, in which disrupted sleep makes the patient drowsy during the day.