Li Y, Sahakian BJ, Kang J, Langley C, Zhang W, Xie C, Xiang S, Yu J, Cheng W, Feng J. The brain structure and genetic mechanisms underlying the nonlinear association between sleep duration, cognition, and mental health. Nat Aging. 2022 Apr 28. Nature Aging

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  1. Previous studies had shown that seven hours is a kind of magic number when considering ideal hours of sleep in the population. The contribution of this paper is the incredible number of individuals included, longitudinal follow-up, and multiple types of data. This allowed the authors to subdivide the group (by age, for instance) and also minimize artifactual measurement variations that always happen when individuals are tested by different professionals and using various machines. As others have done on a smaller scale, this work shows that poor sleep tends to precede cognitive decline.

    That said, this study is not suitable to answer the question of whether [poor] sleep causes cognitive decline or if underlying factors cause both. Studies in humans suggest that AD pathology affects sleep-modulating areas before affecting memory-related areas, supporting the latter hypothesis. Experimental studies suggest a bi-directional feeding mechanism between worse sleep and worse cognition.

    The bottom line, in my opinion, is that some therapies under development for neurodegenerative diseases, especially AD and LBD, will also likely help treat sleep disturbances in adults, because these symptoms have overlapping pathological mechanisms. Because the glymphatic system is more active during sleeping, some argue that improving sleep, even if artificially, may prevent or delay the spreading of AD pathology. Further studies on humans are needed to answer this question.

    View all comments by Lea T. Grinberg

  2. This paper builds on previous work showing that short and long self-reported sleep duration is associated with poorer cognitive performance and worse measures of mental health such as anxiety and depressive symptoms. Similar non-linear relationships between sleep and cognitive performance have been shown in prior work, including our recent paper in Brain (Lucey et al., 2021), using objective sleep measures such as sleep duration, time in different sleep stages, and average non-rapid eye movement slow wave activity.

    Li et al. leverage the richly characterized participants from the UK Biobank study to extend these findings to include mental health symptoms and how these relationships may be mediated by changes in different brain regions and genetic factors. Many of the conclusions in this paper need to be replicated, but the findings have implications for using sleep as a marker and/or potential therapeutic intervention in AD and other brain disorders. 

    Sleep duration likely has a “sweet spot” that has the greatest benefit for brain health, and this paper points to potential mechanisms that warrant further investigation. Future research is needed to establish how these mechanisms mediate the relationship between sleep duration, cognition, and mental health, as well as if they change with age.

    References:

    Lucey BP, Wisch J, Boerwinkle AH, Landsness EC, Toedebusch CD, McLeland JS, Butt OH, Hassenstab J, Morris JC, Ances BM, Holtzman DM. Sleep and longitudinal cognitive performance in preclinical and early symptomatic Alzheimer's disease. Brain. 2021 Oct 22;144(9):2852-2862. PubMed.

    View all comments by Brendan Lucey

  3. It is well known that short and long sleep are both associated with lower-than-average cognitive function, although it is important to stress that effect sizes are quite modest in observational studies. It is impossible to know what causes what, and it is likely that both are caused by other variables. For instance, many types of brain pathology and changes in normal aging affect both cognitive function and sleep, but it is not clear whether the two are directly and causally related.

    Still, this study confirms previous studies showing that shorter sleep than the often-recommended seven to eight hours does not necessarily come with lower cognitive function or smaller brain volumes.”

    View all comments by Anders Martin Fjell

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