Among 869 older adults (mean age 70.7) in the Baltimore Longitudinal Study of Aging, both subjective insomnia symptoms and actigraphy-measured sleep disruption independently pointed to the same gut bacterium: Eubacterium sp. CAG:251. Frequent trouble falling asleep (≥5 nights/week) and excessive daytime sleepiness were associated with depleted or undetected levels of this species. Objectively, each doubling of sleep efficiency raised CAG:251 prevalence 2.15-fold, while each additional 30 minutes of wake-after-sleep-onset halved its prevalence (PR=0.49). Overall microbiome diversity metrics showed weak, inconsistent associations.
The convergence of both self-reported and actigraphic sleep measures on a single bacterial taxon is methodologically compelling — it substantially reduces the risk that findings reflect measurement bias alone. Eubacterium species are short-chain fatty acid producers, and CAG:251 specifically has been flagged in gut health research for its anti-inflammatory potential, though its precise mechanistic role remains poorly characterized. That global diversity metrics showed little signal while species-level analysis revealed a specific target reinforces an emerging paradigm: broad alpha/beta diversity indices may obscure functionally meaningful, taxon-specific sleep-gut relationships.
Limitations are real: this is cross-sectional and observational, causality cannot be established, the actigraphy sub-cohort (n=332) is modest, and the explained variance in beta diversity was small. Still, for a primarily White, older cohort, the biological specificity here is notable. Confirmatory longitudinal or interventional work targeting CAG:251 abundance through sleep improvement protocols would meaningfully advance this field.