Why do I keep falling asleep during the day?

Patient: Almost every day I get a bout of sleepiness. Suddenly, without feeling tired beforehand, my eyes becomes heavy and my head begins to fall forward. I am normally just working on a computer or reading. Today I was holding my phone and just dropped it out of my hand. No matter how hard I try I cannot keep awake. After struggling like this for a while, eventually the symptoms pass. I have no trouble sleeping at night, falling asleep within 5 minutes and get a minimum 7.5 hours sleep a night. What could be causing this?

Symptoms: Sleepiness, Unable to keep awake, heavy eyes