A day sleep promoting role of phototransduction in Drosophila melanogaster.

The daily sleep-wake cycle is a conserved behaviour defined by locomotion quiescence and enhanced responsive threshold to sensory stimuli. Both the circadian clock and sleep-homeostasis determine the daily sleep profile. Environmental light is a major sensory input and also regulates circadian clock and the balance between sleep and wakefulness. In Drosophila, the cellular mechanism and neural circuitry underlying light-mediated circadian synchronization are well-established, yet the direct relationship between light/visual input and sleep remains unclear. To address this knowledge gap, we measured sleep behaviour in Drosophila with mutations in genes involved in phototransduction and downstream neural transmission. We observed consistent day sleep fragmentation in flies with mutations in multiple phototransduction components. We also found that mutation that led to hyperpolarised Drosophila photoreceptors resulted in shorter day sleep. We found a severe reduction in locomotor speed in several visual mutants during normal waking time preventing assessment of their sleep-linked immobility. Taken together, our rigorous quantification of sleep in phototransduction genetic mutants reveals the key role of visual input in promoting sleep.