Lack of sleep harms memory

Stressful, sleepless nights may impair learning and memory retention, new research suggests.

Researchers say stressed-out rats kept up for three days straight failed to produce adequate new cells in the brain's hippocampus - an area thought necessary for learning and memory.

"The stressful nature of sleep deprivation exerts negative effects on the hippocampus," concluded a team led by Elizabeth Gould, a professor of psychology at Princeton University in Princeton, New Jersey.

Her team published the findings in the November 27 early online edition of the Proceedings of the National Academy of Sciences.

How the study was conducted
Gould's group already knew that extended sleep deprivation is a strong stressor for the brain. In this new study, they wondered if the stress hormone corticosterone might play a role in sleep deprivation's effects on new brain cell production.

In the experiment, rats were placed on a circular metal platform suspended just above water in a plastic container. Falling asleep caused the animals to fall into the water - adding heightened stress to the sleep-deprivation mix.

The researchers found that after three days of this type of stressful sleep deprivation, the rats' brains showed elevated levels of corticosterone. Higher levels of the hormone were associated with a significant reduction in the number of new brain cells produced in the rat's hippocampus.

To double-check that link, the researchers repeated the experiments in rats that had had their hormone-producing adrenal glands surgically removed. In these animals, brain-cell production continued as usual, even after similar sleep-deprivation experiments.

Stress hormone tied to brain cells
These results suggested that increases in levels of the stress hormone account for impaired cell production in sleep-deprived brains.

One expert isn't sure whether the effect seen in this study is transferable to people, however.

"A lot of these effects may be species-specific," said Jerry Siegel, a professor of psychiatry at the University of California, Los Angeles. "In the rat, there is data indicating all sorts of dire effects from sleep deprivation leading to death. These are not seen in humans. So, generalising to humans is not easy to do," he said. "There might be a connection, but you can't be sure."

Not necessarily stressful
Another expert isn't sure that the study proves that all sleep deprivation is necessarily stressful.

"There is no doubt that the protocol used here was stressful," said Joseph D. Miller, an associate professor of cell and neurobiology at the University of Southern California. "However, this is not so clear with other methods of sleep deprivation," Miller said.

"For instance, continuous, gentle handling of mice for six hours or so has been reported to cause near maximal increase in sleep propensity once the handling is terminated," he said. "But is this stressful? In an animal habituated to handling, this may be the least stressful way to sleep-deprive."

This could have a parallel in humans - for example, the difference between partying for two or three nights straight (less stressful) or being forced by someone else to stay awake (very stressful).

"Conceivably, you might see an even larger number of surviving neurons if sleep deprivation is as non-stressful as possible," Miller explained. "It might be that sleep deprivation is not all that bad in humans if the associated stress can be reduced." – (HealthDayNews)

Read more:
Sleep Centre
Stress Centre

November 2006

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