Disrupted sleep – the link between ageing and memory decline

Structural brain changes, disrupted sleep, and impaired memory function have each been independently associated with ageing. However, a study recently published in Nature Neuroscience by  Bryce Mander and colleagues suggests that these phenomena are causally linked: structural brain changes associated with ageing interfere with sleep quality, which in turn impairs memory retention. Such findings may have important applications in preventing age-related cognitive decline!

During sleep our memories are consolidated. That is, fresh memory traces in the brain are strengthened and become integrated with existing knowledge. Memories thereby grow more persistent and are less likely to be forgotten. Previous research has shown that sleeping for a period immediately after a learning task has a positive influence on memory retention compared to spending the same amount of time awake.

The medial prefrontal cortex, the most frontal region of the brain, is involved in maintaining a good quality of sleep – it plays a particularly important role in a stage of sleep known as non-rapid eye movement (nREM) slow wave sleep. This region of the brain loses volume with age, and this has been thought to provide an explanation for age-related disruptions in slow wave sleep (SWS), important for memory consolidation. However, the study conducted by Mander’s group is the first to suggest a causal influence of structural brain change on sleep quality and consequently on long-term memory retention.

A group of young adults and a group of older adults performed a learning task and were later exposed to two recognition tasks – one 10 minutes after learning (short-delay) and the other after 8 hours of sleep (long-delay). Brain activity was recorded during sleep in order to monitor the level of slow wave sleep. Furthermore, all participants also had their brain structure assessed.

It was found that prefrontal cortex volume correlated with quality of slow wave sleep in both the young and the older group. Age, therefore, is not independently related to disruptions in the quality of sleep. Rather, poor slow wave sleep is a more direct result of reductions in volume of certain brain areas, particularly the medial prefrontal cortex. These are structural changes that commonly occur with age.

When it came to memory retention, Mander’s team found that the young group performed better on the memory tasks than the older group. For both groups, there was a decline in memory performance for the long-delay condition. However, the memory loss during sleep for the older group was greater than for the young group: that is, the memory benefit of sleep was much larger for the young than for the old participants.

In order to see whether memory deterioration was a result of poor quality slow wave sleep, the researchers compared the sleep patterns to the level of forgetting overnight. These data revealed that the people with worse slow wave sleep showed greater overnight memory loss. Disruption of slow wave sleep was proportional to the deficits in memory retention. This suggests that slow wave sleep, mediated by the medial prefrontal cortex, is responsible for the level of memory consolidation overnight.

These findings have important implications for all age groups, but for the elderly in particular. Preventing age-related structural brain changes is unfeasible, but improving quality of sleep by either exercise or medication could prevent the cognitive decline associated with ageing, especially that related to memory.

Reference: Mander et al. (2013). Prefrontal atrophy , disrupted nREM slow waves and impaired hippocampal-dependent memory in ageing. Nature Neuroscience, doi: 10.1038/nn.3324