Researchers have shed light on how humans fall into a slumber, by observing how special nerve cells in fruit flies cause drowsiness.

The team at Oxford University’s Centre for Neural Circuits and Behaviour (CNCB) worked with a small cluster of neurons that had previously been shown to put fruit flies to sleep when activated.

It was unclear, however, how activation of these neurons triggers sleep behaviour. The team found that, when activated, these sleep-promoting cells control the flow of electrical signals through a key node in the brain.

This in turn controls all the key aspects that induce sleep: the shutting down of the fly’s locomotive system, preventing the animal from sleep walking; the increasing of sensory arousal thresholds, so the fly is not awoken by every small stimulus from its surroundings; and the clearing of the ‘sleep debt’ or tiredness that had accumulated during waking.

Diogo Pimental, a researcher on the project, described the process as a system where the body builds up enough fatigue to pass a “tipping point”, prompting sleep. He said: “The whole system resembles the repeated filling and emptying of a container with liquid.

“Tiredness is the liquid; sleep-control neutrons determine whether the container is horizontal or tilted; and being awake or asleep corresponds to alternating phases of filling or emptying.”

Professor Gero Miesenboeck, Director of the CNCB, noted that while the study discovered how a “beautifully simple system keeps sleep need and sleep in the balance”, unanswered questions remain.

“We still don’t know why sleep debt builds up, what it consists of physically, how it triggers the switch to sleep and how the accumulated sleep debt is cleared. Finding the answers will help us solve the mystery of sleep.”

The CNCB is an autonomous research centre within the University, which aims to further understanding of how intelligence emerges from the physical interaction of nerve cells.

The full paper, ‘Recurrent Circuitry for Balancing Sleep Need and Sleep’, can be read in the journal, Neuron.

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