Your friend has experienced excessive daytime sleepiness. he is laughing with you and suddenly falls to the ground. your friend is probably suffering from

I believe the answer is: Narcolepsy
Narcolepsy refers to a neurological disorder that make the victim experienced excessive daytime sleepiness.
It's very common for the victim of this disorder's brain to suddenly entering a sleep mode and resulted in uncontrollable episodes of falling asleep during the daytime. 

Ithink d hope that