Monday January 28 2013
Slow wave activity during sleep lowers as we age
"Boosting sleep 'may slow memory rot'," says the BBC, in a headline that could provide us with a plausible excuse to give our bosses when we fall asleep at our desks.
The news is based on a complex study that involved testing younger and older people's memories before and after sleep. Researchers gave participants word pairs to remember in the evening, tested them on half of the word pairs before sleeping, and the other half of the words after they had slept.
Older adults were found to have smaller grey matter volume in an area of the brain called the prefrontal cortex, which is involved in generating slow brain waves in young adults during sleep.
The researchers' analysis suggests that some of the observations they made in this study may be linked, rather than being three independent effects of ageing: prefrontal changes could be related to altered sleep patterns, and changes in sleep patterns could be related to memory impairment.
But the study's authors are very guarded about their findings, stressing that they have not firmly established that this chain of events is what causes memory impairment.
Overall, this research furthers our understanding of what happens to our brain and memory as we age, and how this might relate to brain activity during sleep. We can't say from this study alone whether interventions to improve sleep in older people would have a beneficial effect on their memory, or if this could reduce or prevent reduction in brain performance (cognitive decline).
Where did the story come from?
The study was carried out by researchers from the University of California and other research centres in California. It was funded by the US National Institutes of Health (NIH).
It was published in the peer-reviewed journal Nature Neuroscience.
BBC News' coverage of the study is accurate and appropriately includes a note of caution from Dr Simon Ridley, speaking on behalf of the charity Alzheimer's Research UK. Dr Ridley says that, "Increasing evidence has linked changes in sleep to memory problems and dementia, but it's not clear whether these changes might be a cause or consequence."
What kind of research was this?
This was an experimental study assessing whether sleep disruption in older people could contribute to the changes in our cognitive abilities as we age – for example, having problems remembering things.
The researchers report that in healthy young people, a certain period of sleep called non-rapid eye movement (NREM) slow wave sleep is thought to improve people's ability to remember new pieces of information.
They say that as we age, a number of things happen:
- certain areas of the brain get smaller
- there are reduced levels of NREM slow wave sleep
- long-term memory becomes impaired
However, it is not clear whether these factors are linked or if they contribute to cognitive decline in later life. The researchers wanted to test whether this might be the case.
They carried out short-term experiments as an initial test of their hypothesis. More long-term research is needed to assess the relationship between sleep and cognitive decline – whether one leads directly to the other, or if separate factors affect both.
What did the research involve?
The researchers enrolled a group of 15 "cognitively normal" older adults (average age about 75 years) and 18 healthy young adults (average age about 21 years).
People with disorders affecting the brain, psychiatric disorders, depressive symptoms, sleep disorders or taking medication which could affect their brain function were excluded. To be included in the study, participants had to be within what was considered to be a normal range for their age group in cognitive tests.
The researchers asked participants to perform a memory task involving associating random word pairs consisting of real words such as "bird" and nonsense words such as "jubu".
They were trained to remember the word pairs in the evening before going to sleep, and were tested on some of the words 10 minutes after the training. They then had an eight-hour sleep period where their sleep and brain waves were monitored. They were tested on the remaining word pairs in the morning, while their brains were scanned using a functional MRI to look at activity in areas of the brain associated with memory retention.
Functional MRI scanning tracks the flow of blood in the brain. Increased blood flow in certain areas of the brain corresponds with increased neural activity, so the scans show which parts of the brain are more active at any given time.
After the test, the participants had structural MRI brain scans to measure different parts of their brains' grey matter. "Grey matter" is brain tissue that contains the bodies of the nerve cells.
The researchers also carried out the same experiment a second time, but without a sleep period between the tests.
They compared the overnight memory performance of the older and younger participants, and looked at whether their sleep brain activity and brain structural measurements related to their memory performance.
What were the basic results?
The older adults showed less slow wave brain activity during sleep than younger adults, including in a part of the brain called the pre-frontal cortex (PFC). Older adults also spent less time in slow wave sleep than younger ones and showed less grey matter tissue volume than younger adults, particularly in the PFC region.
The researchers found that older age was statistically associated with less slow wave brain activity during sleep and lower grey matter volume in the PFC.
Lower grey matter volume in the PFC was also associated with less slow wave activity during sleep, and age did not have a significant effect on slow wave brain activity once PFC grey matter volume was taken into account. This was not the case with other brain regions.
The researchers interpreted these results as suggesting that the relationship between age and slow wave activity is linked to a reduction in grey matter in the PFC.
Poorer performance in pre-sleep to post-sleep memory testing was greater in older adults. There were similar findings if both memory tests were performed in the day, with no sleep between tests. However, younger people performed significantly better if the two tests were separated by a sleep period, while older people did not.
More slow wave activity (particularly in the PFC region) was associated with better overnight memory performance in older and younger adults.
Both age and PFC grey matter volume were associated with the amount of slow wave activity during sleep, as well as with overnight memory performance. However, age and PFC grey matter volume did not have a significant relationship with overnight memory performance once slow wave activity during sleep was taken into account.
The researchers suggest that the relationship between age and PFC grey matter volume is affected by the amount of slow wave activity during sleep.
How did the researchers interpret the results?
The researchers concluded that their results support the hypothesis that consolidation of episodic memories (remembering new pieces of information) deteriorates with age, partly caused by shrinking grey matter in the PFC region of the brain. This is also linked to reductions in NREM slow wave sleep.
They note that this data alone cannot establish that one factor causes the other – for example, that reduced slow wave activity directly causes impaired memory retention in older adults.
The authors say that their findings support the possibility that treatments that improve slow wave sleep in older adults could potentially reduce the cognitive decline associated with poor long-term memory in later life. They refer to other studies that have already tried to do this using either drugs or non-pharmacological means.
Conclusion
The experiments in this complex study have suggested that age-related shrinking in a specific part of the brain (the prefrontal cortex) is linked to impairment in older adults' abilities to remember new information, and that this link could be related to how much short wave brain activity older people have during sleep.
The authors themselves are very cautious about their findings, noting that they cannot directly establish that this chain of events is what causes memory impairment in older people.
For example, it is difficult to determine from this study whether poorer quality sleep causes changes in the brain which then cause memory problems, or whether changes in the brain affect sleep quality, and so on.
Overall, this research furthers our understanding of what happens to our brains and memory as we age, and how this might relate to brain activity during sleep.
We can't say from this study whether changing brain activity in sleep in older people would have an effect on their memory or cognitive decline, but it sounds as if these researchers plan to test whether it could in further studies.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.