How animals maintain proper amounts of sleep yet still be flexible to changes in the environmental conditions remains unknown. Here we showed that environmental light suppresses the wake-promoting effects of dopamine in fly brains. A subset of clock neurons, the 10 large lateral-ventral neurons (l-LNvs), are wake-promoting and respond to dopamine, octopamine as well as light. Behavioral and imaging analyses suggested that dopamine is a stronger arousal signal than octopamine. Surprisingly, light exposure not only suppressed the l-LNv responses but also synchronized responses of neighboring l-LNvs. This regulation occured by distinct mechanisms: light-mediated suppression of octopamine responses is regulated by the circadian clock, whereas light regulation of dopamine responses occurs by upregulation of inhibitory dopamine receptors. Plasticity therefore alters the relative importance of diverse cues based on the environmental mix of stimuli. The regulatory mechanisms described here may contribute to the control of sleep stability while still allowing behavioral flexibility.
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