The Influence of Caffeine on <i>Siesta</i> and Nighttime Sleep in <i>Drosophila melanogaster</i>

Caffeine, a plant-derived psychostimulant, has been demonstrated to reduce sleep and increase dopaminergic neuron activity in mammals through competitive antagonism at adenosine receptors (AdoRs). However, the extent to which it influences daytime sleep (siesta) as much as nighttime sleep and the involvement of the adenosine signaling pathway in this process remain unsolved. It is similarly unclear whether the influence of caffeine varies with age and depends on sex and what type of cells it affects most; the clock cells that are involved in sleep timing and regulation or dopaminergic neurons, which are crucial for voluntary movement. To address this question, the role of caffeine has been investigated in Drosophila melanogaster (wild-type Canton-S flies), which has also been observed to reduce sleep in response to caffeine. The siesta and nighttime sleep of 3-, 30-, and 50-day-old males and females of Canton-S Drosophila were examined. Furthermore, transgenic flies with overexpressed or silenced dAdoR in all neurons (elav-expressing cells), the circadian clock cells (tim-expressing cells), and dopaminergic neurons (th-expressing cells) were studied. We found that females exhibited greater sensitivity to caffeine than males, and that older flies slept longer during the day than young flies. However, caffeine treatment resulted in reducing siesta in flies with overexpression of dAdoR in all neurons, tim-, and th-expressing cells. Conversely, silencing of dAdoR increased siesta. Therefore, the observed differences in the daytime sleep of Drosophila appeared to depend on signaling through AdoRs.