Sleep debt has become a common phrase in health news recently, as many different research studies have shown that insufficient amounts of sleep can have lasting negative impacts on health.

A new report published in the journal Nature provides some of the first evidence of how the brain changes its activity in response to sleep deprivation.

Dr. J. Gillian and colleagues at the University of California San Diego have used a technique called functional magnetic resonance imaging (fMRI) to measure the amount of activity occurring in different areas of the brain. The group compared the brain activity of people who were deprived of sleep for 35 hours to that of people who had received their normal amount of sleep. The researchers found that in tests of verbal learning and memory, sleep-deprived subjects showed a different pattern of brain activity than was seen in the brains of people after a normal amount of sleep. Sleep-deprived subjects also tended to perform more poorly on the verbal memory tests than did the rested subjects.

Sleep-deprived subjects showed increased activity in the parietal lobes and the prefontal cortex, whereas the temporal lobes (the part of the brain near the ears) were more active in the rested subjects. The researchers found that the people who reported feeling the most sleepy showed the most activity in their parietal lobes.

The researchers express surprise in their paper, stating that they had expected to see a decrease in brain activity rather than a shift in the areas of activation. Their results indicate that the brain is able to shift its patterns of activity in response to changes in the body’s physiological state, and that those changes could contribute to the impaired verbal learning seen in the sleep-deprived subjects.

The fact that sleep deprivation leads to a change in the pattern of brain activity, rather than simply decreasing activity, provides new insight into the way the brain processes information. One interpretation of the results from this study is that without sleep, the normal verbal learning centers of the brain are unable to function, and in order to complete verbal learning tasks the sleep-deprived brain relies on “backup” areas which are not as efficient with learning and memory.

It should be noted that this study does not definitively show that sleep deprivation is a direct cause of the changes in brain activity. For example, sleep deprivation could change the output of a particular hormone, and it is this change in hormone levels that might cause the change in brain activity patterns. What the group in San Diego has shown us is that there does seem to be a distinct correlation between certain patterns of brain activity and sleep deprivation.

Many recent studies have reported psychological and physiological effects of sleep deprivation. Dr. William C. Dement, a sleep researcher at Stanford University, has published a book called “The Promise of Sleep” in which he describes the concept of sleep debt-the idea that the body requires a specific amount of sleep and suffers when this amount is not fulfilled. Some of the penalties of a prolonged sleep debt include a depressed immune system and an increased risk of high blood pressure and diabetes. Dr. Dement has stated in newspaper interviews that alleviating the burden of sleep debt could save thousands of lives every year.

Another study conducted by sleep scientists at the University of Chicago has found that sleep debt caused alterations in the body’s metabolic and endocrine functions, and that these physiological changes resemble some of the hallmark changes associated with aging.

As our ever-fast paced world increases our level of sleep debt, knowledge of the underlying neurological effects of sleep deprivation becomes increasingly relevant. The study from the group in San Diego provides some of our first clues to the physical changes that can occur in the brain when it has not received the appropriate amount of rest.