Christian Holland

The function of sleep is one of the enduring mysteries of modern neuroscience. Despite years of concerted research, it is still unclear why all known animals undergo a regular period of unconsciousness and what function this fulfils that simply cannot be performed during wakefulness. Key to identifying this function will be an understanding of the sleep homeostat. This internal bookkeeper measures the need for sleep that accumulates during wakefulness and dissipates during sleep. It therefore directly measures the processes that require sleep. This essay presents evidence that the sleep homeostat in Drosophila induces sleep through the activation of a group of 24 neurons projecting to the dorsal fan-shaped body (dFB) and identifies several sources of input to these effector neurons. I propose that this discovery allows a crucial reframing of the mystery of sleep as a molecular question and suggest experiments by which the dFB neurons can be used to evaluate existing theories of sleep. While current evidence is insufficient to support any one particular theory, this essay argues that future work investigating how the dFB is activated will lead to an understanding of the processes that require sleep and, thus, provide a window on the vital function of sleep.