The brain really can be half asleep, says research

Moments of absent mindedness such as losing your keys could be the result of tiny parts of the brain taking "naps" to recharge, a study finds.

Researchers discovered that contrary to popular opinion the brain is not always entirely asleep or awake but parts of it can go "offline". This they say 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 team at the University of Wisconsin discovered that some nerve cells in tired but awake individuals can briefly go "offline". Professor Chiara Cirelli, a psychiatrist and author of the study, said: "Even before you feel exhausted, there are signs in the brain that you should stop certain activities that may need attention. "Specific groups of neurons may be falling asleep, with negative consequences on performance."

Until now, scientists thought that sleep deprivation generally affected the entire brain. EEGs that measure electrical activity in the brain, show network can detect whether neurons in the brain are on or off. Prof Cirelli said: "We know that when we are sleepy, we make mistakes and our attention wanders. "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.

The researchers inserted probes into specific groups of neurons in the brains of freely-behaving rats. After the rats were kept awake for longer periods, the probes showed areas of "local sleep" despite the animals' appearance of being awake and active. Prof Cirelli said: "Even when some neurons went offline, the overall EEG measurements of the brain indicated wakefulness in the rats."

She said there were behavioural problems caused by local sleep episodes. "When we prolonged the awake period, we saw the rats start to make mistakes," she said. When animals were challenged to do a tricky task, such as reaching with one paw to get a sugar pellet, they began to drop the pellets or miss in reaching for them, indicating that a few neurons might have gone offline.

Prof Cirelli said: "This activity happened in few cells. "For instance, out of 20 neurons we monitored in one experiment, 18 stayed awake. From the other two, there were signs of sleep—brief periods of activity alternating with periods of silence."