Momentary absent mindedness may be the result of tired parts of the brain taking a quick nap, say scientists.

Certain nerve cells can go into an “off-line” dormant state while the rest of the brain remains awake, research has shown.

The result can be a misplaced set of keys or the cereal ending up in the refrigerator.

“Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness,” said lead researcher Chiara Cirelli, professor at the University of Wisconsin-Madison in the US. Specific groups of neurons may be falling asleep, with negative consequences on performance.”

Previously scientists thought sleep deprivation generally affected the whole brain, and short periods of “micro-sleep” were believed to be the most likely cause of falling asleep at the wheel.

But the new research, published in the journal Nature, suggests that even before the “micro-sleep” stage, brains may be impaired by sleep-like activity.

The scientists made the discovery after inserting probes recording electrical activity into groups of neurons in the brains of freely-behaving rats.