No one knows the exact cause of Idiopathic Hypersomnia. That's what the word "idiopathic" means, that the exact cause is unknown. But research suggests that the quality of nighttime sleep could play an important role.
Low amounts of the deepest (slow-wave) phase of sleep
This is the kind of sleep that is restorative and helps you feel refreshed when you wake up
Frequent interruptions (arousals) during sleep
Interruptions in your sleep cycle when shifting from a deep phase of sleep to a light one may cause your sleep to feel less refreshing
The levels of a natural chemical in your body that may play a role in decreasing brain activity (consciousness) and regulating sleep
A network in the brain that’s key to alertness and sleep
How your nervous system communicates with your body
Your circadian rhythm, or "internal clock"