Most Common Cause of Excessive Daytime Sleepiness

What is the most common cause of excessive daytime sleepiness?

Sleep deprivation
Narcolepsy
Drug effects
Depression

Comments: If u mean "Jet lag" from sleep deprivation, then the common cause is narcolepsy. It really depends on age. In children the most common cause is nightmares, in young people it can both be a psychiatric disorder or a manifestation of their daytime routine, in middle age and elderly it is mostly Obstructive Sleep Apnea Syndrome.

Correct Answer: 

Sleep deprivation