This can occur when you are either immersed in something else, or just so bored that your brain has decided to take a nap.
Often affects people on medication, alcohol or illegal drugs, but can happen to anyone.
"Wha' . . . Ger'um'ah, yeah."
"What the hell have you been smoking? He's shaking like a head-knocker."
"Um . . . your stash. Sorry man got a little Brain Clog today."