It’s no accident that we spend a third of our lives asleep. It is essential to our health, and even animals for whom resting is complicated – such as aquatic mammals that need to surface to breathe, or birds that go up to 10 days without touching dry land – manage to sleep with surprising adaptations.

But while we sleep, the tens of trillions of microorganisms that live within us – known as the microbiota – follow their own rhythms. This microscopic colony, composed mainly of bacteria, can weigh as much as 200g. Together with its host, it forms a biological unit known as the holobiont.

The microbiota is not just a passenger. Within each person’s body it comprises a functional unit, one that influences vital processes such as digestion, immune function and, as we will see here, sleep.

Our relationship with the microorganisms we harbour is also deeply interdependent. Not only do we provide them with a place to live, but we also depend on them to maintain numerous biological functions. This delicate balance is increasingly being linked to health, wellbeing and, quite possibly, to longevity.

Read more: What the gut microbiome of the........

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