We all experience moments when we wonder “what were we thinking?” Often, the event is an embarrassing action that occurs when we are tired.

New research on rats suggests that in such cases a part of the brain was literally taking a very quick nap.

Researchers from the University of Wisconsin-Madison have discovered some nerve cells in a sleep-deprived, yet awake brain, can briefly go “off line,” into a sleep-like state while the rest of the brain appears awake.

“Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness,” says Dr. Chiara Cirelli, professor of psychiatry at the School of Medicine and Public Health.

“Specific groups of neurons may be falling asleep, with negative consequences on performance.”

Traditionally, scientists have believed the entire brain is affected by sleep deprivation – a premise developed from the observance that electroencephalograms (EEGs) show brain-wave patterns typical of either being asleep or awake.

“We know that when we are sleepy, we make mistakes, our attention wanders and our vigilance goes down,” says Cirelli. “We have seen with EEGs that even while we are awake, we can experience shorts periods of ‘micro sleep.’ ”

Periods of micro sleep were thought to be the most likely cause of people falling asleep at the wheel while driving, Cirelli says. The new research suggests that even before that stage, brains are already showing sleep-like activity that impairs them.

As reported in the current issue of Nature, the researchers inserted probes into specific groups of neurons in the brains of 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.

“Even when some neurons went off line, the overall EEG measurements of the brain indicated wakefulness in the rats,” Cirelli says.

And there were behavioral consequences to the local sleep episodes.

“When we prolonged the awake period, we saw the rats start to make mistakes,” Cirelli says.

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 off line.

“This activity happened in few cells,” Cirelli adds. “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.”

Source: University of Wisconsin-Madison