Sleep 'promotes memory-related brain changes'

"The mechanism by which a good night's sleep improves learning and memory has been discovered by scientists," is the somewhat overeager reporting on the BBC News website. While the study had intriguing results, it only involved mice.

This study in mice looked at if and how sleep helps memory and learning. The researchers got mice to perform a running task, running both forwards and backwards on a rotating rod.

Some of the mice were allowed to sleep afterwards and some were sleep deprived. The mice were then microscopically examined to see how sleep, or the lack of it, influenced the connections between the nerve cells in the brain.

Mice who were allowed to sleep experienced the formation of new dendrites (spinous projections at the end of nerve cells), which pass electrical signals from one nerve cell to another.

The formation of new dendrites may be linked to changes in the brain that are associated with learning and experience (changes to the brain's plasticity). But if the mice were deprived of sleep, these new connections did not develop.

The study may not have a direct application to humans, however, and much of the mystery of sleep remains to be discovered. But this research could be another small piece of the puzzle that suggests that sleep is one of the ways we may help consolidate our learning.

Where did the story come from?

The study was carried out by researchers from the New York University School of Medicine in the US and Peking University Shenzhen Graduate School in China, and was funded by the US National Institutes of Health, a Whitehall Foundation research grant, and a grant from the American Federation for Aging Research.

The study was published in the peer-reviewed journal, Science Magazine.

The main body of the BBC News reporting is accurate, though the headline "Sleep's memory role discovered" is probably far too definitive for the limited, if interesting, research outlined in the study.

What kind of research was this?

This study in mice aimed to see how sleep helps memory and learning. The researchers did this by getting mice to perform a running task.

Some of the mice were allowed to sleep afterwards, while some were sleep deprived. The mice's brains were then microscopically examined to see how sleep influenced the connections between the nerve cells in the brain.

The researchers say that it is believed that sleep has an effect on the connections (synapses) between nerve cells, which are important for the formation of memories. But the role that sleep plays in learning and experience-dependent changes to the synapses is said to be unclear.

What did the research involve?

This study aimed to see how performing a running task remodels the spinous projections (dendrites) that form the connections between nerve cells, and then see how this was influenced by sleep.

A group of mice were trained to run forwards on an accelerated rotating rod. The spinous projections connecting the nerve cells in the brain were microscopically examined before and after the task.

These mice were compared to a group of mice who had not received the rotating rod training. The researchers then looked at the effect of training the mice to run either forwards or backwards on the rod.

After these tests, the researchers examined the potential role of sleep in the process. They compared mice who were allowed to sleep in the seven hours after a period of forward running on the rod with mice who were sleep deprived by continuous gentle handling for seven hours after the task.

They then tested whether the effect of sleep deprivation could be "rescued" by allowing the mice to sleep in the 16 hours after the initial seven hours of sleep deprivation.

What were the basic results?

The mice had an increased development of new spinous projections between nerve cells in the brain in the 24 hours after the rotating rod task when compared with mice who had not done the task.

When they repeated the running task, this time allowing the mice to run forwards or backwards, the researchers found that backwards running led to the formation of a different set of spinous projections.

The researchers then compared mice who were allowed to sleep after the rotating rod task with mice who were sleep deprived. They found that the sleep-deprived mice showed a significantly reduced formation of new spinous projections between nerve cells.

Even if the sleep-deprived mice were allowed to train on the rotating rod for double the amount of time beforehand, it made no difference – they still demonstrated fewer nerve cell connections than the mice who were allowed to sleep.

The 16-hour recovery sleep after the initial deprivation had no effect – they had fewer projections, suggesting the effects of sleep deprivation could not be "rescued".

For mice who had been allowed to sleep, the projections between the nerve cells were demonstrated to persist in the following days, supporting the common understanding that a skill is learned and persists for long periods of time with minimum interference from other learning.

When looking at specific sleep stages, the researchers found that deprivation of REM sleep (the deepest period of sleep in which dreams are thought to occur) in particular did not diminish the synapses. This suggests that non-REM sleep may be involved in forming new nerve connections after learning.

How did the researchers interpret the results?

The researchers conclude that their findings indicate that sleep has a key role in promoting learning-dependent connections between nerve cells, which contribute to memory storage.

Conclusion

This research in mice furthers our understanding of the important role that sleep plays in consolidating our learning and memory.

When mice learned to run on a rotating rod, the task led to the formation of new spinous projections (dendrites) at the end of nerve cells, which pass electrical signals from one nerve cell to another.

However, if the mice were deprived of sleep, these new connections did not develop.

This effect could not be "rescued", regardless of whether they were allowed to train for a much longer period before being sleep deprived, or if they were allowed to sleep for a long period after their initial sleep deprivation.

The researchers also found that most of the changes to the nerve cell connections seemed to be occurring during non-REM sleep, rather than REM sleep.

The results of this study may not have a direct application to humans. But if further evidence proves that it does, it suggests that attempting to compensate for the adverse effects of missing sleep, such as sleeping for 16 hours after pulling an all-nighter, may be futile: The mice were unable to "rescue" the beneficial effects to the brain if they were sleep deprived for a significant amount of time.

Known harmful effects of not getting enough sleep include:

detrimental effects on mood, such as irritability
impaired cognitive function
increased vulnerability to infection