By Alan Z. Segal, MD
Associate Professor of Clinical Neurology, Weill Cornell Medical College
Dr. Segal reports no financial relationships relevant to this field of study.
SYNOPSIS: In a series of clinical studies of cognitively normal people older than 60 years of age, disruption of REM sleep and reduced quantity of REM sleep increased the risk of developing Alzheimer-type dementia.
SOURCE: Pase MP, Himali JJ, Grima NA, et al. Sleep architecture and the risk of incident dementia in the community. Neurology 2017; Aug. 23. pii: 10.1212/WNL.0000000000004373. doi: 10.1212/WNL.0000000000004373. [Epub ahead of print].
In last month’s Neurology Alert, we reviewed a landmark study in the journal Brain, showing that disruption of slow-wave sleep led to increases in cerebrospinal fluid amyloid-β levels. The implication was that this deep phase of non-REM sleep plays a unique role in cleansing the brain of toxic substances that otherwise would promote the development of dementia. No relationship between REM sleep and amyloid was found.
The current study, published on the heels of the Brain report, has nearly opposite findings. Polysomnography was performed in 321 subjects from the Framingham offspring study who were older than 60 years of age at the time of sleep assessment. In 12 years of follow-up, 32 cases of incident dementia (primarily Alzheimer’s type) were recorded — the majority of these during the latter stages of follow-up. For each percentage reduction in the quantity of REM sleep, there was a 9% increased risk of dementia. Four times as many subjects in the lowest quartile of REM duration developed dementia when compared to subjects in the highest quartile of REM sleep. In direct contradiction to the Brain report, there was no relationship found between slow-wave sleep and incident dementia. Furthermore, REM loss was not thought merely to be a marker of mental decline, since any patient with mild cognitive impairment or dementia onset within the first three years of follow-up was excluded.
Although REM sleep may play a role in promoting synaptic plasticity and the consolidation of memories, it remains speculative as to how REM sleep may protect against dementia. Although there is muscular relaxation during REM, the activity of the cortex more closely approximates the waking state, in contrast to the downregulation of brain activity observed in slow-wave sleep. Despite the apparent contradictions in these two important studies, there is little doubt that sleep in general, whether slow-wave or REM, plays a crucial homeostatic role, not only in our daily function but also in our long-term brain health. It is important to maintain good sleep habits with advancing age to reduce the risk of developing Alzheimer’s disease and other dementias.