As the pace of life quickens many people meet their challenges by getting less sleep. However, scientists have now found that this has serious consequences for the brain and for the learning abilities. Even more interestingly, they have found that the learning process actually creates new brain cells.
It is long known that lack of sleep impairs cognitive function. Sleep-restricted individuals have a shorter attention span, impaired memory, and a longer reaction time. But why is sleep important? The scientists are now beginning to understand better what actually happens in the brain: Learning spatial tasks increases the production of new cells in an area of the brain involved with spatial memory called the hippocampus. Sleep plays a part in helping those new brain cells survive.
The team of researchers from the University of California and Stanford University found that sleep-restricted rats had a harder time remembering a path through a maze compared to their rested counterparts. And unlike the rats that got enough sleep, the sleep-restricted rats showed reduced survival rate of new hippocampus cells. (The researchers used sleep-restricted rats rather than sleep-deprived rats to mimic more closely the common human experience of inadequate sleep during the work week.)
Previous studies have shown that the hippocampus is important for spatial learning. "The hippocampus also has the unique ability to generate new brain cells throughout life, a process
called 'neurogenesis,'" noted Ilana Hairston the author of the study. "When animals learn a task that requires the hippocampus, the rate of neurogenesis increases. This suggests that learning itself rejuvenates the brain."
Knowing that spatial learning triggers production of new brain cells in the hippocampus, Hairston and her team wanted to find out whether restricting sleep during a spatial learning task would also affect new cell production in the hippocampus.
The researchers used a water maze: rats were placed in the water and had to swim to the exit platform.
One group or rats could not see the platform, which was placed underwater, and had to form a "mental map" of the maze -- a spatial memory task that is hippocampus-dependent -- to quickly reach the exit.
The second group could see and smell the exit platform, which had a citrus odor. The researchers moved the platform every fourth trial, thus requiring the animal to rely on its senses, not on memory, to find it. This task did not engage the hippocampus because the rat did not need a mental map of the pool to reach the platform.
At the end of each training session, half the animals in each group were kept awake for six hours by being presented novel stimuli that kept them interested and awake. The other half were returned to their cages and allowed to sleep. After six hours, the sleep-restricted rats were allowed to sleep for the remainder of the day until the next session, 18 hours later.
The result was that rested rats that had to rely on memory manifested increased neurogenesis, while the sleep restricted rats showed no signs of increased neurogenesis. This means that lack of sleep undoes the cell rejuvenation benefit that would normally come from the task, the researchers noted.
On the other hand, the researchers were surprised to discover that sleep-restricted rats that had to rely on their senses to find the exit did a better job than the rested rats that had to rely on their senses.
Hairston and her colleagues believe it is because the rested group tried to rely on memory to find the platform, generally a better strategy to reach a goal you have reached before. But in this case, where the researchers moved the goal every fourth trial, using the visual and odor cues was a better strategy. It appears that the sleep-restricted rats changed their strategy to compensate for their lack of sleep - and it worked.
"The sleep-restricted rats in this group actually did better because the lack of sleep interfered with their ability to memorize the maze -- forcing them to rely on easily accessible clues," Hairston said. "This finding could be used to design training regimens for chronically sleep-deprived people, including members of the military and medical students. 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."
One implication of these findings is that sleep restriction disrupts the hierarchy of cognitive processes. That is, spatial learning seemed to be the primary cognitive strategy, and only when it was disrupted by lack of sleep did a secondary strategy emerge. "It would be interesting to expand our findings to see if other competing processes are similarly affected by sleep restriction," Hairston said.
For example, scientists know that people who have suffered certain types of brain lesions may be unable to screen out irrelevant stimuli such as random noises in a room, something healthy individuals do easily. A flip side is that people with these lesions tend to associate familiar stimuli with new information more rapidly than healthy counterparts, a phenomenon called attention switching.
This suggests that learning to ignore stimuli and rapid attention switching are competing processes, with healthy individuals ignoring familiar stimuli as their primary strategy. "It would be interesting to assess whether sleep restriction causes people to lose the ability to screen out extraneous stimuli and preferentially apply attention switching", she said.