Idiopathic hypersomnia is an uncommon sleep disorder that causes you to be excessively sleepy during the day even after a good or prolonged night's sleep. It also often causes difficulty waking up after you've been asleep at night or for a nap. Naps generally aren't refreshing.
The need to sleep can strike at any time, including when you're driving a car or working, which makes idiopathic hypersomnia potentially dangerous.
The condition often develops over weeks to months. Diagnosing idiopathic hypersomnia requires ruling out more common sleep disorders. Treatment is aimed at controlling symptoms with medication.