We’ve all experienced it. That moment of wandering attention. The semi-blank stare. You can’t remember that question you were going to ask. It’s the “zoning out” phenomenon. It happens more frequently when you are fatigued or sleep-deprived. What is really happening in your head when you go blank? Your’re neither asleep nor fully awake.

The University of Wisconsin at Madison has been able to shine a little light on this problem. While we are awake, neurons in the cerebral cortex fire irregularly, which results in rapid fluctuations on an EEG. As we sleep, the brain alternates periods that look like an awake EEG with periods where the neurons stop firing altogether.

The researchers monitored activity in focal areas of rat brains (tough to extrapolate this one). They found that after prolonged sleep deprivation, small random areas of the brain would switch off and look like they were asleep. However, the animal appeared to be awake, and the EEG looked like that of an awake rat. If neurons switched off in the motor strip while tasks were being performed, performance errors increased dramatically.

The lead investigator believes that these “tired neurons” may be responsible for attention lapses, poor judgment, mistakes and irritability when we haven’t gotten enough sleep, but don’t feel sleepy. This phenomenon may represent a global state of neural instability, and individual neurons switch off to save energy or restore themselves.

Reference: Nature 472:443-447, April 27, 2011.