Study: Sleep-deprivation may impair memory

McGraw Center tutors may be right after all: Getting a good night's sleep before an exam appears to bode better for performance than staying up all night to cram.

In a recent study by psychology professor Elizabeth Gould, rats who were sleep-deprived for 72 hours exhibited increased levels of the stress hormone glucocorticoid. These high stress levels in turn reduced neurogenesis — the birth of new neurons — in the rats' hippocampuses, a part of the brain critical for learning and memory.

Though the findings pertain only to rats, they are in line with previous research on impaired performance in sleep-deprived humans and may implicate a new mechanism for this impairment.

"It's possible that suppression of neurogenesis could contribute to some cognitive functions associated with sleep deprivation," Gould said in an interview.

To study the effect of sleep deprivation on neurogenesis, Gould and her colleagues placed rats on a platform where they could sit comfortably. When they started to fall asleep and lose muscle tone, the rats would fall into a container of water, waking them up and forcing them climb back onto the platform.

In this way, the rats were kept awake for 72 hours. Then the researchers sacrificed the animals to make their measurements, discovering that the rats showed increased glucocorticoid levels and decreased neurogenesis.

When the researchers removed the rats' adrenal glands to eliminate stress hormones, neurogenesis continued, even after sleep deprivation.

In other words, it's the stress of sleep deprivation, not just the lack of sleep, that reduces neurogenesis and may impair cognitive function.

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"It's a stress effect," Gould said.

These results imply that sleep deprivation alone won't necessarily hurt cognitive functioning.

Since "there are differences in the degree to which sleep deprivation is stressful," some sleep-depriving activities may be more harmful than others, Gould said. Sleep deprivation due to positive social activity, for instance, would be less stressful than sleep deprivation resulting from fear. In the first case, neurogenesis might not be affected, so learning and memory might remain intact.

Though there is no evidence in humans that sleep deprivation decreases neurogenesis, the findings may shed light on how lack of sleep could impair human performance.

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"Stress has also been shown to reduce adult neurogenesis" in "nonhuman primates as well as rodents," Christian Mirescu, a postdoctoral researcher who helped write the study, said in an email. So it "seems plausible that it does this in humans as well."

But, Mirescu added, "For there to be any potential impact of impaired adult neurogenesis on cognitive function, I think extended impairments in cell birth would be required," not just the short-term impairments in the paper.

So will a good night's sleep lead to neurogenesis and therefore better learning and memory?

Gould cautioned that sleep is not the opposite of sleep deprivation. Just because sleep deprivation reduces neuron production does not necessarily mean that sleep increases it, she said. Indeed, sleep deprivation is just one of many activities that can trigger stress hormones.

But even if sleep's effects on neurogenesis are inconclusive, Gould said, it's still a good idea to get some rest. Being sleep-deprived, she explained, is just "not a good way to live."