SAN FRANCISCO — Sleep-deprived people make mistakes. New research suggests that a tired brain may turn on the equivalent of an internal screen saver instead of concentrating on mental tasks, which may explain those blunders.

Ninad Gujar of the University of California, Berkeley and colleagues presented evidence March 22 at the annual meeting of the Cognitive Neuroscience Society that sleep deprivation affects the brain’s default network.

Scientists describe the default network as the parts of the brain that deactivate when a person is doing a specific mental task, such as having a conversation, reading or memorizing a list of words, or solving a math problem. The network is active, though, when people are ruminating, daydreaming, recalling the past or when the mind wanders. Researchers still don’t fully understand how the network works and how it affects cognition.

Gujar wanted to find out if sleep deprivation affects the default network. His team tested 28 young adults; half got a normal amount of sleep and half were kept awake for 35 hours before testing. The volunteers did memory tests while in an fMRI scanner. Every time the volunteers saw a particular picture, they were supposed to punch a button.

But Gujar was more interested in what happened in the volunteers’ brains while they waited for the picture to pop up. He left enough time between the correct picture and other images to allow participants’ brains to slip into default mode, and then he looked at brain activity.

In default mode, sleep-deprived volunteers displayed less activity than those who had enough sleep in the anterior cingulate cortex and also displayed an overactive precuneus, key parts of the default network. Gujar was able to predict with 93 percent accuracy whether a volunteer was sleep deprived just from the activity pattern in the two areas. The sleep-deprived pattern in those two areas was also associated with more misses on the memory tests.

Gujar says that the pattern he saw in sleep-deprived people suggests that information is not flowing correctly in the brain’s default network. He speculates that the default network could be turning on or remaining on when sleep-deprived volunteers are trying to concentrate on mental tasks, which could explain the misses.

The study gives more information about how the default network works and indicates that sleep is essential for its proper functioning, says Damien Fair, a cognitive neuroscientist at the Oregon Health & Science University in Portland. More research is needed to confirm the Berkeley team’s findings, he said.

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