I keep falling asleep when sitting for long periods of time, it's almost like my body is shutting down and it's so hard to stay awake once it starts.?

Falling asleep: Falling asleep can be a sign of many things. Narcolepsy is diagnosed by having a sleep specialist (usually a neurologist) do a thorough interview and then measure brainwaves during a series of naps taken in the sleep lab ("mulitple sleep latency test") narcolepsy can cause people to fall asleep while driving, so it is important to know if you have it. It is also highly treatable.