You have narcolepsy?
lol, but in all seriousness you are probably draining yourself. Do you just zone out and hit the floor? Or have to sit down and you fall asleep? Try to gather your energy up for a bit, even use energy from outside your body, before casting. And make sure you've grounded and centered and shielded and all that good stuff, before and after.
Sleeping seems to be natural defense to stuff; when I get upset or worried I suddenly become very, very tired and I have to sleep. And I'll sleep all day in broad daylight, noise, everything. Maybe it's a blood pressure thing. You may be freaking yourself out? Is it sleeping or fainting?