Excessive daytime sleepiness is otherwise known as hypersomnia.
There are a variety of primary sleep conditions that can cause this symptom. These include such conditions as narcolepsy, idiopathic hypersomnia, obstructive sleep apnea, periodic limb movements of sleep, and behaviorally induced insufficient sleep. Of course, this symptom can also be caused by medications and chronic medical conditions. A thorough sleep evaluation as necessary to assess the cause.