Sleepyheads’ brains veer from restful path

A good night’s sleep seems to clear the brain and help the well-snoozed individual negotiate the flow of daily affairs.

Sleep matters for brain activity. Christopher Bissell/Tony Stone

In contrast, a lack of sleep makes it difficult to carry out even mundane acts, such as conversing intelligibly or calculating a waiter’s tip. Initial investigations of brain activity in sleepy volunteers as they try to perform verbal and mathematical tasks have yielded intriguing clues about the nature of sleep deprivation.

The new evidence adds to scientific concern over the potentially harmful effects of widespread sleep deficits (SN: 9/25/99, p. 205: Brain functions tapped during certain types of thought change considerably after a sleepless night, reports a team of neuroscientists led by Sean P.A. Drummond of the University of California, San Diego.

As they try to memorize words, sleep-deprived adults exhibit a pattern of brain activity not previously recorded, Drummond and his colleagues say. This response may reflect the recruitment of a brain system to compensate, at least in part, for neural losses sparked by sleepiness, they suggest.

In contrast, sleep-deprived people display generally lower brain activity while grappling with math problems than they do when rested, says the team. In this case, no new areas of heightened neural activity compensate for the effects of sleepiness.

“The effects of sleep deprivation on cognitive performance and related patterns of cerebral activation may depend in part on task-specific demands,” the researchers conclude.

During both tasks, functional magnetic resonance imaging scans measured blood-flow changes in specific brain regions. These data provided an indirect index of rises and falls in brain-cell activity.

In one study, published in the Feb. 10 Nature, the researchers evaluated 13 healthy adults, ages 21 to 35, in a sleep laboratory. Each participant tried to memorize short lists of words on the afternoon following a full night’s sleep and then after about 35 hours without sleep. Word recall and recognition dropped sharply in the sleep-deprived.

Parts of the prefrontal cortex and temporal cortex showed the most activity in rested individuals, the scientists say. The prefrontal cortex helps coordinate attention and memory for information used in various tasks. The temporal cortex contributes to language comprehension.

The scientists had presumed that a lack of sleep would obstruct prefrontal activity, which increases sharply during wakefulness. Contrary to expectations, however, the prefrontal cortex after sleep deprivation exhibited even more activity across a larger area as volunteers studied the lists.

Sleepy participants also displayed blood-flow surges in another brain area, the parietal lobe, but not in the temporal cortex.

The buildup of sleep-promoting substances, such as adenosine, in the prefrontal cortex of sleepy individuals may cause that area to work harder when confronted with a verbal task, Drummond and his coworkers theorize.

In another study, published in the Dec. 16, 1999 NeuroReport, participants performed a series of arithmetic tasks that required subtraction. Parietal regions that were activated in rested participants exhibited blood-flow declines after 35 hours without sleep.

Activity in other brain regions remained stable or declined slightly while sleep-needy volunteers worked on the math problems. Participants made more mistakes and omitted more responses when drowsy.

“This is exciting work,” remarks sleep researcher David Dinges of the University of Pennsylvania School of Medicine in Philadelphia. “But brain processes involved in sleep deprivation are probably more complex than any current scientific explanations.”

Alterations of brain activity in sleepy people may reflect the use of new cortical areas to compensate for neural losses elsewhere, Dinges says. However, the observed brain changes may instead result, in part, from impairment of the ability to sustain attention for more than a few seconds, he adds.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.