Insomnia

Which Way the Influence Actually Runs

This question has been approached with a method that can separate cause from coincidence, and the answer is not the flattering one. Insomnia means persistent difficulty falling asleep or staying asleep, with daytime consequences. People with insomnia do have different gut bacteria from people without it. The interesting work has gone into asking which came first, and the results suggest the sleep problem is doing more of the pushing than the bacteria are.

Borrowing genetics to test cause

Comparing two groups of people cannot separate cause from consequence. Researchers get round this using a method called Mendelian randomisation, which uses inherited genetic variants as a natural experiment.

The logic is simple. Genetic variants are fixed at conception and shuffled at random between siblings. If variants that happen to raise the abundance of a particular bacterium also raise insomnia risk, the bacterium is more plausibly contributing to the insomnia than the reverse, because insomnia cannot alter the genes a person was born with.

One study applied this to genetic data on insomnia from 386,533 people, alongside two large bacterial genetic datasets covering 18,340 and 8,208 people.

Fourteen bacterial groups appeared to raise insomnia risk. Eight appeared to lower it. But the effect sizes were tiny. The risky groups raised the odds of insomnia by roughly 1% to 4% each, and the protective ones lowered them by 1% to 3%.

Source: Investigating bidirectional causal relationships between gut microbiota and insomnia

The traffic in the other direction is far heavier

The same study ran the analysis backwards, testing whether genetic liability to insomnia predicted bacterial abundance.

Here the numbers were much larger. Insomnia was associated with raising the abundance of 12 bacterial groups, with odds ratios between 1.65 and 4.43. It was associated with reducing 7 others, with odds ratios between 0.21 and 0.57.

Compare those figures with the ones going the other way. Bacteria shifted insomnia odds by a few per cent. Insomnia shifted bacterial abundance by factors of two, three and four.

That asymmetry is the honest headline. Poor sleep appears to reshape the gut community substantially. The gut community appears to affect sleep only slightly.

The mechanisms are not mysterious. Sleep loss disturbs the daily rhythm that gut bacteria follow, alters when and what people eat, and raises stress hormones that change conditions in the gut.

Where the studies disagree

Different research groups running similar analyses have not landed in the same place, which matters when judging how firm any of this is.

A separate study using the same approach found the family Ruminococcaceae associated with higher insomnia risk, with odds of roughly 1.49, and the genus Lachnospiraceae with odds of about 1.73. Both are large groups of fibre fermenters usually described as beneficial.

Another analysis identified ten bacterial groups with apparent causal links to insomnia, and a different six affected in reverse.

Very little overlap exists between the specific bacteria named in these studies. When three teams use similar methods on overlapping data and produce three different lists, the honest reading is that the broad relationship is real while the details are not yet settled.

Source: A bidirectional Mendelian randomization study investigating the causal role between gut microbiota and insomnia

What this means for you

Persistent insomnia is worth taking to a doctor, and cognitive behavioural therapy for insomnia is the recommended first treatment. In addition, the genetic work above shows broken sleep thinning parts of the gut community, which makes protecting it through a bad stretch worthwhile. A varied, plant-rich diet supports the same gut bacteria that broken sleep appears to thin out. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.