The JAMA Neurology study was observational and therefore could not prove that too little or too much sleep causes cognitive problems.
Notes on verification
Multiple independent sources (PubMed, JAMA Network, Alzheimer Europe, and a peer-reviewed citing paper) confirm the study's cross-sectional/observational design and the standard methodological limitation that such designs cannot establish causation.
Sources
- How much sleep keeps cognitive decline at bay? - Harvard Health (health.harvard.edu)
- https://pubmed.ncbi.nlm.nih.gov/34459862/ (pubmed.ncbi.nlm.nih.gov)
- https://www.alzheimer-europe.org/news/new-research-jama-neurology-explores-impact-sleep-duration-amyloid-accumulation-and-cognitive?language_content_entity=en (alzheimer-europe.org)
- https://jamanetwork.com/journals/jamaneurology/fullarticle/2783664 (jamanetwork.com)
- https://pmc.ncbi.nlm.nih.gov/articles/PMC13343368/ (pmc.ncbi.nlm.nih.gov)