Mental Blankness

The brain is said to be working all the time, thinking through the days and night. Then why do I enter a state of pure blankness, nothing to think, nothing remotely inside my brain, not even in the vicinity. At these precise moments, instinctively, I start to mumble something, to make others believe that I have thoughts of grave importance crossing through my brain, but the actual thought is that what the hell am I mumbling and why the fuck am I thoughtless.