Losing Sleep Undoes the Rejuvenating Effects New Learning Has on the Brain

As the pace of life quickens and it becomes harder to balance home

and work, many people meet their obligations by getting less sleep.

But sleep deprivation impairs spatial learning -- including

remembering how to get to a new destination. And now scientists are

beginning to understand how that happens: 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.

A 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 more closely mimic the common human experience of inadequate

sleep during the work week, said lead investigator Ilana Hairston of

both the University of California, Berkeley, and Stanford University.

The paper, "Sleep restriction suppresses neurogenesis induced by

hippocampus-dependent learning," appears in the Journal of

Neurophysiology published by the American Physiological Society.

Stanford researchers Milton T.M. Little, Michael D. Scanlon, Monique

T. Barakat, Theo D. Palmer, Robert M. Sapolsky, and H. Craig Heller

co-authored the paper.

Learning appears to rejuvenate the brain

Scientists already know -- and most of us can confirm from firsthand

experience -- that lack of sleep impairs cognitive function. Sleep-

restricted individuals have a shorter attention span, impaired

memory, and a longer reaction time. "Sleep is necessary for general

health, but it now appears that the brain needs sleep more than any

other part of the body," Hairston said.

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,'" Hairston noted. "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 affect new

cell production in the hippocampus.

The experiment: swimming to the exit platform

The researchers trained rats on one of two tasks using a water maze

-- a plastic pool about six feet in circumference and two feet deep.

Rats were placed in the water and had to swim to the exit platform.

One group 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,

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, Hairston explained.

Fewer brain cells for the weary

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.

Rested animals that had to rely on memory to find the goal showed

increased neurogenesis in the hippocampus compared with animals that

could use sight and smell. That made sense, because the task that

relied on memory involved the hippocampus, while the other did not.

However, the sleep-restricted rats that had to rely on memory to find

the goal showed no increased neurogenesis, unlike their rested

counterparts. This means that lack of sleep undoes the cell

rejuvenation benefit that would normally come from the task, the

researchers noted.

Sleep restriction prompts use of a secondary strategy

On the other hand, the sleep-restricted rats that were required to

locate the platform using visual and odor cues did better on the task

than their rested counterparts. This was an unexpected finding.

Hairston et al. 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 cues," Hairston said.

Researchers point to practical implications for the overtired

Overall, the study underlined that learning depends upon two things:

exposure to novel material and getting a good night's sleep, Hairston

said.

Learning new things, at least in the case of spatial memory, quite

literally keeps your brain young by ensuring a better survival rate

for new brain cells in the hippocampus. However, not getting enough

sleep eliminates the potential benefit of new learning on the

hippocampus by suppressing neurogenesis. "Mild, chronic sleep

restriction may have long-term deleterious effects on neural

functioning," according to the paper.

On the other hand, that sleep-deprived rats did better on a task

requiring use of visual and odor cues compared to their better rested

counterparts "implies that some kinds of cognitive function are

resistant to sleep loss," Hairston said. "This may be significant in

human learning as well, and implies that it may be possible to

optimize the way information is presented to rested versus fatigued

individuals to take advantage of the specific neural substrates that

are unaffected by sleep loss," the researchers concluded.

"This finding could be used to design training regimens for

chronically sleep-deprived people, including members of the military

and medical students," Hairston said. "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," according to the paper.

Further studies could clarify learning strategies the brain employs

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.