The ability to be synchronized by light-dark cycles is a fundamental property of circadian clocks. Although there are indications that circadian clocks are extremely light-sensitive and that they can be set by the low irradiances that occur at dawn and dusk, this has not been shown on the cellular level. Here, we demonstrate that a subset of Drosophila's pacemaker neurons responds to nocturnal dim light. At a nighttime illumination comparable to quartermoonlight intensity, the flies increase activity levels and shift their typical morning and evening activity peaks into the night. In parallel, clock protein levels are reduced, and clock protein rhythms shift in opposed direction in subsets of the previously identified morning and evening pacemaker cells. No effect was observed on the peripheral clock in the eye. Our results demonstrate that the neurons driving rhythmic behavior are extremely light-sensitive and capable of shifting activity in response to the very low light intensities that regularly occur in nature. This sensitivity may be instrumental in adaptation to different photoperiods, as was proposed by the morning and evening oscillator model of Pittendrigh and Daan. We also show that this adaptation depends on retinal input but is independent of cryptochrome.circadian rhythm ͉ dual-oscillator model ͉ PERIOD ͉ synchronization ͉ TIMELESS E ndogenous circadian clocks prepare organisms according to the most reliable and predictable of environmental changes, the cycle of day and night. To function as reliable timers, circadian clocks themselves are synchronized to the 24-h cycle. This synchronization is accomplished mainly by light. Whereas light during the day has little effect on circadian clocks, they are most susceptible to light in the early and late night. Bright light pulses applied during the early night delay the phase of the clock; light pulses during the late night advance it (1). Stable synchronization occurs when the delaying and advancing effects of light on the clock in the early (at dusk) and late night (at dawn) are of equal strength. In nature, stable synchronization is a challenging task, because irradiances during dawn and dusk can vary largely from day to day because of the weather. Bünning (2) measured irradiances systematically throughout day and night and found that day-to-day fluctuations are smallest during early dawn and late dusk, when the irradiances are still Ͻ10 lux. Therefore, he proposed that organisms time their clocks to the very low irradiances occurring during early dawn and late dusk and thus must be very light-sensitive. Indeed, he found that bean plants synchronize to light of moonlight intensity (0.6-0.8 lux) and that artificial moonlight applied during the night phase shifted the rhythm of leaf movement (2). Bean plants lower their leaves during the night, and Bünning suggested that they need to do so to decrease the moonlight reaching the leaf surface to avoid their light-sensitive clocks interpreting the moonlight as the coming dawn (2).In animals, it is under deb...

Artificial moonlight was recently shown to shift the endogenous clock of fruit flies and make them nocturnal. To test whether this nocturnal activity is partly due to masking effects of light, we exposed the clock-mutants per(01), tim(01), per(01);tim(01), cyc(01), and Clk(JRK) to light/dark and light/dim-light cycles and determined the activity level during the day and night. We found that under moonlit nights, all clock mutants shifted their activity significantly into the night, suggesting that this effect is independent of the clock. We also recorded the flies under continuous artificial moonlight and darkness to judge the effect of dim constant light on the activity level. All mutants, except Clk(JRK) flies, were significantly more active under artificial moonlight conditions than under complete darkness. Unexpectedly, we found residual rhythmicity of per(01) and especially tim(01) mutants under these conditions, suggesting that TIM and especially PER retained some activity in the absence of its respective partner. Nevertheless, as even the double mutants and the cyc(01) and Clk(JRK) mutants shifted their activity into the night, we conclude that dim light stimulates the activity of fruit flies in a clock-independent manner. Thus, nocturnal light has a twofold influence on flies: it shifts the circadian clock, and it increases nocturnal activity independently of the clock. The latter was also observed in some primates by others and might therefore be of a more general validity.