The brain is not always entirely asleep or awake.
Contrary to popular opinion the brain is not always entirely asleep or awake but parts of it can go “offline”, researchers have discovered.
This they claim 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, who measured electrical waves in the brain, discovered that some nerve cells in tired yet awake individuals can briefly go “offline”.
Professor Chiara Cirelli, a psychiatrist and author of the study, 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.”
Until now, scientists thought that sleep deprivation generally affected the entire brain. Electroencephalograms (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, 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 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.
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.”
The researchers tested only motor tasks, so they concluded from the study published in the journal Nature, that neurons affected by local sleep are in the motor cortex.
This is the area of the central brain which plans, executes and controls movement and the carrying out of tasks.