This Gut Bacterium Vanishes in Older Adults Who Can’t Sleep

If you've spent restless nights wondering whether your gut is somehow tangled up with your sleep, a new study out of the Baltimore Longitudinal Study of Aging (BLSA) gives that hunch some real backing. Researchers tracked sleep patterns and stool samples from hundreds of older adults and found that people who struggled to fall asleep or felt persistently drowsy during the day were consistently missing more of one particular gut bacterium than everyone else. The finding doesn't prove the bacterium causes insomnia, but it puts a name and a number on a link that has mostly stayed vague until now.

Key takeaways

• Researchers analyzed gut bacteria and sleep in 869 older adults (average age 70.7) and found that both self-reported insomnia symptoms and device-measured sleep efficiency pointed to the same depleted microbe: Eubacterium sp. CAG:251.
• People with frequent trouble falling asleep actually had higher, not lower, overall bacterial diversity, an unexpected twist that complicates a simple "more diversity is better" story.
• The study is cross-sectional (a single snapshot in time), so it cannot say whether poor sleep depletes this bacterium, its absence worsens sleep, or something else drives both. The authors themselves call the findings exploratory, pending replication.

Eubacterium CAG:251 Depletion Linked to Insomnia

A new analysis published August 8, 2026 in the journal SLEEP found that levels of a specific, little-studied gut microbe, Eubacterium sp. CAG:251, drop in older adults who report frequent insomnia symptoms and excessive daytime sleepiness. The research team, led by Curtis Tilves and Adam Spira at Johns Hopkins along with collaborators from the National Institute on Aging, drew on data from the long-running BLSA cohort. They compared self-reported sleep questionnaires and, in a subset of participants, wrist-worn actigraphy devices (motion trackers that estimate sleep timing and efficiency) against shotgun metagenomic sequencing (a method that reads nearly all the DNA in a stool sample to identify which bacterial species are present and in what proportions). People who reported trouble falling asleep on five or more nights a week and those with excessive daytime sleepiness both showed depleted Eubacterium sp. CAG:251, and so did people whose actigraphy-measured sleep efficiency was lower. That convergence, subjective complaints and objective device data landing on the same bacterium, is what makes this particular result stand out from the broader, messier literature on gut bacteria and sleep.

Why Poor Sleep and Gut Bacteria May Be Linked

Anyone managing chronic insomnia, especially in later life, has a stake in figuring out what else is going on in the body when sleep breaks down. Insomnia in older adults is already tied to worse mood, slower cognition, and higher risk of falls, and researchers have spent the last decade probing whether the gut microbiome, the trillions of bacteria living in the digestive tract, might be a two-way street with sleep. Some animal work suggests gut bacteria can influence sleep-regulating signals, while poor sleep in turn can alter gut bacterial populations. This study doesn't resolve that direction question, but it narrows the search: instead of a vague "gut health affects sleep" claim, there is now a specific bacterial species and a specific measured relationship (PR=2.15, meaning roughly double the likelihood of detecting the bacterium in people with better sleep efficiency) to investigate further.

How Nearly 900 Older Adults Were Studied

The credibility of this finding rests on its scale and its use of two independent ways of measuring sleep. The researchers analyzed 869 participants with both self-reported sleep data and metagenomic sequencing, and a smaller group of 332 of those also wore actigraphy devices. Participants averaged 70.7 years old, were 54.8% female, and were predominantly White (66.9%). The team used statistical methods including PERMANOVA (a test for whether groups of bacteria differ as a whole) and ANCOM-BC2 (a method for pinpointing which individual bacterial species differ between groups), and they adjusted their results for age, sex, BMI, physical activity, education, and depressive symptoms, factors that could otherwise muddy a sleep-microbiome link.

What the Finding Means for Your Gut and Sleep

For a reader wondering what to do with this, the honest answer right now is: not much, yet. This study does not establish that eating or supplementing your way to more Eubacterium sp. CAG:251 would improve sleep, and no such intervention has been tested here. What it does offer is a concrete lead worth watching. If you have chronic insomnia or persistent daytime sleepiness, it may be worth mentioning to a doctor if you're also managing a gut-related condition, since this is one of the more specific gut-sleep signals to emerge from a well-powered human study rather than an animal model. The unexpected finding that frequent trouble falling asleep tracked with higher Shannon diversity (a standard measure of how many different bacterial species are present and how evenly distributed they are, up 0.41 standard deviations, 95% CI 0.09 to 0.73) is a reminder that "more diverse gut bacteria" isn't automatically the same as "healthier gut bacteria" in every context.

Why These Findings Are Still Exploratory

The biggest limitation is built into the study's design: it's cross-sectional, meaning everyone was measured at a single point in time, so the researchers cannot say whether poor sleep caused the bacterial depletion, the bacterial depletion contributed to poor sleep, or some third factor, like inflammation or diet, drives both. The authors themselves describe their results as exploratory pending longitudinal replication. Notably, actigraphy-measured sleep and self-reported sleep did not agree with each other especially well when it came to which bacteria they were linked to; no actigraphy variables were associated with overall bacterial diversity at all, even though self-reported insomnia was.

What Comes Next for Gut-Sleep Research

The logical next step, which the authors explicitly call for, is a longitudinal study that follows the same older adults over months or years, tracking both sleep and gut bacteria repeatedly to see which change comes first. Researchers will also likely want to test whether Eubacterium sp. CAG:251 has any plausible biological mechanism for affecting sleep, for instance through the compounds it produces, before anyone can responsibly talk about targeting it therapeutically. For now, this remains a well-designed but early-stage signal from a single cohort.

Common Questions

What is Eubacterium sp. CAG:251? It's a species of gut bacteria that has been identified mainly through large-scale DNA sequencing rather than traditional lab culturing, which is why relatively little is known about what it does; this study is among the first to link it specifically to human sleep patterns.

Does this mean poor sleep is caused by gut bacteria? Not according to this study. Because the design is cross-sectional, it cannot distinguish between gut bacteria affecting sleep, sleep affecting gut bacteria, or another factor influencing both.

Should I take a probiotic to fix insomnia based on this study? No. The study did not test any supplement, probiotic, or dietary intervention, and no such approach has been shown to restore this bacterium or improve sleep as a result.

Why did higher bacterial diversity show up in people with worse sleep? The researchers aren't certain; it runs counter to the common assumption that more bacterial diversity is always better, and it's one of several reasons they describe the overall diversity findings as inconsistent, even as the Eubacterium sp. CAG:251 result held up across both self-reported and device-measured sleep problems.

References

1. Curtis Tilves, Calliope Holingue, Sarah K Wanigatunga, et al. (2026). Associations of self-reported and actigraphic sleep with gut microbiome composition and diversity among older adults. SLEEP. https://doi.org/10.1093/sleep/zsag217
2. SLEEP journal advance article. academic.oup.com. https://academic.oup.com/sleep/advance-article/doi/10.1093/sleep/zsag217/8756962