[quote=@rush99999]
[@Guardian Angel Haruki]

Just read the rules regarding the Sleep spell. Does not being able to breathe not count as taking damage?
[/quote]

From what I've read of the rules for drowning, it surprisingly doesn't! 

Because with drowning, the rule is literally "[i]When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can’t regain hit points or be stabilized until it can breathe again,[/I]"

So, no actual damage is dealt to the character like an attack or spell would deal damage. It's just you fall unconscious and start dying.