London: Losing your keys or misplacing your spectacles could be the result of tiny parts of the brain taking `naps` to recharge.
Researchers discovered the brain is not always entirely asleep or awake but parts of it can go `offline.`
This accounts for the feeling of being "half asleep" which causes forgetfulness and small errors such as misplacing keys or putting the milk in the cupboard or the cereal in the fridge, the journal Nature reports.
The team at the University of Wisconsin, US, which measured electrical waves in the brain, discovered that some nerve cells in tired yet awake individuals can briefly go "offline".
Prof Chiara Cirelli, psychiatrist and study author at Wisconsin, said: "Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness.
"Specific groups of neurons may be falling asleep, with negative consequences on performance."
Prof Cirelli said: "We know that when we are sleepy, we make mistakes, our attention wanders and our vigilance goes down.
"We have seen with EEGs that even while we are awake, we can experience shorts periods of `micro sleep`."
She said periods of micro sleep were thought to be the most likely cause of people falling asleep at the wheel while driving.
However, the new research found that even before that stage, brains are already showing sleep-like activity that impairs them.
The researchers inserted probes into specific groups of neurons (nerve cells) in the brains of freely-behaving rats. After the rats were kept awake for prolonged periods, the probes showed areas of "local sleep" despite the animals` appearance of being awake and active.
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