Rodents that got half their normal amount of shut-eye had a harder time remembering how to navigate a maze than well-rested rats, Dr Ilana Hairston of the University of California at Berkeley and colleagues found.

And while new neurons sprouted and survived in a part of the brain associated with spatial learning in the animals that trained in the maze and then slept adequately, this increased growth of brain cells did not happen in the sleep-restricted animals, Hairston and her team reported.

Lack of adequate sleep is "definitely not good for the brain in the long run," the investigator said. "It slows learning."

Researchers had previously shown that limiting sleep impairs learning that depends on the hippocampus, a section of the brain at work in mastering spatial tasks. Past investigators also had demonstrated that when hippocampal learning occurs, the survival of new brain cells there is increased.

Hairston and her colleagues set out to investigate whether slower learning linked with restricted sleep was related to reduced neuron survival.

Rats underwent a four-day training in a water maze. Because the exit was hidden, the animals had to rely on their memories to find their way out. Half of the rodents were kept awake for half of the time that they would normally be asleep -- a condition meant to approximate the low-level sleep deprivation many people experience in daily life.

The animals that slept less learned more slowly, and didn't show increased neuron survival in the hippocampus. But they fared better than the rested rats on a different type of maze task, in which the exit was visible, marked out with a citrus scent, and moved for every fourth run through the maze.

The sleep-deprived animals did better because they relied on their senses, rather than their spatial memories, to solve the maze, Hairston said.

She and her colleagues say this suggests that special techniques could be developed to help chronically sleep-deprived people, such as members of the military or medical students, learn more easily. "That said, while the cognitive impairment may be overcome, our findings indicate that mild, chronic sleep restriction may have long-term deleterious effects on neural function," they concluded.

In other words, Hairston said, "You need both experience and a good sleep afterwards in order to have neurogenesis."

SOURCE: Journal of Neurophysiology