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Memory problems show brain is having a quick rest

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.

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

"Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness," said lead researcher Professor Chiara Cirelli, from 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 affected the whole brain.

Short periods of "micro-sleep" were believed to be the most likely cause of falling asleep at the wheel.

But the new research suggests that even before this stage, brains may be impaired by sleep-like activity.