Scientists Identify Neuronal “Sleep Switch” in Fruit Flies

A group of neurons in the fruit fly (Drosophila) brain send the fly to sleep when they are switched on. This new finding fills a large gap in our understanding of sleep homeostasis, the intrinsic regulatory system which matches sleep intensity and quality to sleep need.

Researchers at the Oxford Centre for Neural Circuits and Behaviour observed that these neurones lit up as the fly slept. In contrast, mutant flies carrying a non-functional variant of these neurons became increasingly sleep deprived, and were unable to compensate for poor sleep duration during the previous cycle. The authors propose that the sleep switch neurons act as an output system for sleep homeostasis as their excitability increases progressively with sleep deprivation.

The sleep-wake cycle is thought to be controlled by two processes. One is a strict molecular body clock, synchronised to day and night length. This is well characterised, and found in insects and mammals alike. The other is sleep homeostasis. The consensus view in this field is that wakefulness builds sleep pressure and sleep builds wake pressure, resulting in a stable system that produces just the right amount of sleep. This is why you get sleepy after a long, strenuous day. Weekend lie-ins reset the sleep homeostat after the lost sleep of a working week. The biological basis of this phenomenon has remained poorly characterised – until now.

The researchers now hope to flesh out the detail of this neural pathway and connect it directly to some form of sleep pressure. The sleep switch may exist in vertebrates and even mammals too, although possibly with embellishments. In the long term, scientists hope that this discovery could drive the development of effective drugs for sleep disorders such as narcolepsy.