New results challenge the view that a good night's sleep can leave behind a dense bloom of brain cells in the morning. Prior studies had found that sleep-deprived rodents grow fewer new neurons than well-rested animals, suggesting that sleep somehow promotes the birth of brain cells, called neurogenesis. But that might not be the case: researchers report instead that lack of sleep likely cuts into neurogenesis by triggering a harmful stress response.

Neurogenesis is a mysterious process that can be amplified by Prozac and other depression treatments, although its exact role in the brain is unclear. Neuroscientist Elizabeth Gould of Princeton University and other researchers have found that many types of stress inhibit neurogenesis in rodents and primates. "We were very curious to see whether the reported effect of sleep deprivation on neurogenesis was related to stress or whether this is something specific to sleep," she says.

To keep rats awake, Gould and her colleagues suspended individual animals above water on a cramped platform for up to 72 hours. The platform would capsize whenever the rodents lost muscle tension, which occurs during deep REM sleep, spilling them into the water and forcing them to scramble back onto their perch.

After 24 and 72 hours, the researchers measured each rodent's rate of neurogenesis and concentration of a stress hormone called corticosterone, which is produced by the adrenal gland near the kidney. As expected, the animals exhibited a decline in neurogenesis after 72 hours but not after only 24 hours. Similarly, their corticosterone levels more than tripled after 72 hours of insomnia compared with that observed after the first 24 hours, meaning hormone levels shift at the right time to influence neurogenesis.

To see if they could erase the change in neurogenesis by keeping hormone levels constant, the researchers removed the adrenal glands from some rats and fed them low levels of corticosterone. When these animals underwent sleep deprivation, they showed no decline in neurogenesis compared with caged rats that slept normally, according to the group's report, published online November 27 in the Proceedings of the National Academy of Sciences USA.

Because it kicks in the stress response, "sleep deprivation is not just the reverse of what you get in the sleep state," Gould explains. "It's a very abnormal condition, and it probably doesn't give us much insight into what sleep does for neurogenesis."

Neuroscientist Dennis McGinty of the University of California, Los Angeles, disagrees. Different methods of sleep deprivation induce varying amounts of stress or even no stress response, and "she's using the most stressful method of sleep deprivation," he says. The result does support the view that neurogenesis is unaffected by REM sleep, he acknowledges. "It's probably not the last word, but it's certainly strongly suggestive."