To the Editor We read with great enthusiasm the fascinating report by Leary et al1 that lower rapid eye movement (REM) sleep amounts are associated with reduced survival, a finding sufficiently robust to withstand adjustments for sleep apnea, abnormal sleep duration, imminent death, and depression and independently replicated in another cohort. The accompanying Editorial2 posed the provocative question of whether this finding might have any practical use to a clinical neurologist. A separate question raised from these data is whether interindividual variability in REM percentage, compared with other sleep features, might provide another clue to REM’s biological importance. As evidenced by coefficients of variation (COV) (SD / mean × 100) for REM percentage within these cohorts, 34.4% for the Outcomes of Sleep Disorders in Older Men (MrOS) Sleep Study and 36.9% for the Wisconsin Sleep Cohort (WSC), differences across individuals in this measure are quite small. By contrast, their magnitude is greatly exceeded by COVs of conditions with known morbidity that neurologists typically treat, such as sleep apnea, for which COVs for the apnea hypopnea index (with a mean calculated across quartiles) were 75.7% and 131.6% for the MrOS and WSC studies, respectively, and for periodic leg movements (often treated when accompanied by restless legs), for which COVs (with a mean calculated across quartiles) were 105.1% and 84.9% for the MrOS and WSC studies, respectively. Apart from disease, perhaps even more striking were the restricted REM COVs in comparison with other prominent sleep architecture features not constituting disease in their own right. For the percentage in N3 (78.4% and 76.6% for the MrOS and WSC studies, respectively), a stage fundamentally constituted by slow waves, the loss of which is often considered to reflect decreased synaptic density and memory consolidation deficits,3,4 COVs were twice as large. The current article1 is not the first report from the MrOS study suggesting that REM, rather than N3, may hold greater significance in this cohort,5 although the finding’s resilience in the WSC study (with similar REM COVs), greatly enhances the likelihood of generalizability.
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Bliwise DL, Trotti LM. Living to Dream. JAMA Neurol. 2021;78(4):495. doi:10.1001/jamaneurol.2021.0053
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