Sleep is a puzzling behaviour that is ubiquitous throughout the animal kingdom, and
while it is implicated in many biological processes, its core function remains elusive.
Although the core advantage sleep provides for an animal is still debated, it is widely
accepted to be regulated by a two-step process that describes the circadian control of
sleep timing, ensuring an organism’s sleep-wake cycle occurs with the passing day and
night and a second homeostatic system which guarantees that animals fulfil their daily
amount and quality of sleep. The circadian aspects of sleep regulation are primarily
understood, but homeostasis is still a relatively mysterious process. In the last 20
years of studying sleep homeostasis in Drosophila melanogaster, a picture of the sleep
circuity and theories of the underlying molecular mechanisms that produce sleep need
have emerged. However, the homeostatic control of sleep is complex, with growing
evidence of multiple circuits that may differentially influence the amount and quality
of sleep determined by the waking environment. Understanding sleep regulation in
other species of the Drosophila genus may elucidate the emergence of sleep homeostasis
in the animal kingdom, utilising their evolutionary distance to compare the shared
fundamental aspects of sleep. In Chapter 3 of this thesis, I employed a comparative
approach in order to establish the circadian and homeostatic elements of sleep behaviour
in a selection of Drosophila species, including D.melanogaster, D.simulans, D.sechellia,
D.erecta, D.yakuba, D.willistoni, and D.virilis. I found a striking disparity in their
neuromodulatory sleep signalling and homeostatic response to a mechanical method of
sleep deprivation, while the circadian timing of sleep was broadly conserved. In Chapter
4, I investigated the underlying mechanisms of the homeostatic response using the tools
available in the D.melanogaster model and elucidated that alterations in their synaptic
strength dynamics may underscore the divergence in homeostasis observed.