If you've ever mislaid your keys or glasses and blamed this on being daft or forgetful, you may be doing yourself a disservice: you may be a bit short of sleep, that's all.
So suggests a study on rats, which says that if a brain is tired, parts of it can fall asleep for a fraction of a second, even though the organ is ostensibly awake at the time.
The implications are far-reaching, especially for people doing tasks where lack of sleep could be dangerous, say its authors.
"Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness," said Chiara Cirelli, a professor of psychiatry at the University of Wisconsin at Madison.
"Specific groups of neurons may be falling asleep, with negative consequences on performance."
The investigation, published in the British journal Nature, challenges conventional belief that sleep deprivation affects the entire brain.
This theory is underpinned by evidence from electro-encephalograms, or EEGs, which show patterns of electrical activity by brain cells, also called neurons.
EEGs, though, have limitations.
Their electrodes are placed on the scalp, which means they are better at picking up signals close to the skull rather than signals from deep within the cranium. And they essentially summarise activity by hundreds of millions of neurons and so cannot pinpoint individual cells.
Cirelli and colleagues inserted ultra-fine probes into the brains of 11 adult rats to monitor electrical activity in sub-groups of neurons in the motor cortex, which is in charge of semi-automatic "motor" movements.
The rodents were then kept up four hours beyond their usual bedtime, thanks to novel objects that were introduced into their cage to keep them awake.
The monitors showed that even when the animals were to all outward appearances awake and active, neurons in these specific areas did not fire - in other words, parts of the brain were in "local sleep" even when others were awake.
"Even when some neurons went off line, the overall EEG measurements of the brain indicated wakefulness," said Cirelli.
Episodes of "local sleep" affected the rats' behaviour, the scientists found.
The animals were trained for two hours to carry out a tricky task, of grapping a sugar pellet with a single paw.
But the more tired they became, the harder they found to do this job. They began to drop the pellets or miss reaching out to them.
It needed just a few cells to go off-line, and for as short as a third of a second, for the fumble to happen, said Cirelli in a press release.
"Out of 20 neurons we monitored in one experiment, 18 stayed awake," she said. "From the other two (neurons), there were signs of sleep - brief periods of activity alternating with periods of silence."
Previous work in sleep research, using EEGs, has highlighted the risk of micro-sleep, when someone can doze for just a matter of seconds. This is often a cause of car accidents.
But the new investigation - assuming the findings on rats can be extrapolated to humans - implies that behaviour can be impaired even before micro-sleep sets in.
In a commentary also published by Nature, Christopher Colwell, a sleep specialist at the University of California at Los Angeles, sounded caution, saying speculation about impact on performance among humans could be an "intellectual stretch."