We investigated the effects of natural light at night (LAN) in the field and artificial LAN in the laboratory on the circadian rhythm of pupal eclosion in a tropical wild type strain of Drosophila jambulina captured at Galle, Sri Lanka (6.1(o)N, 80.2(o)E). The influence of natural LAN, varying in intensity from 0.004 lux (starlight intensity) to 0.45 lux (moonlight intensity), on the entrainment pattern of the circadian rhythm of eclosion at 25(o) +/- 0.5(o)C was examined by subjecting the mixed-aged pupae to natural cycles of light and darkness at the breeding site of this strain in the field. The eclosion peak was approximately 2 h prior to sunrise, and the 24 h rhythmicity was the most robust. Effects of artificial LAN at 25(o) +/- 0.5(o)C were determined in the laboratory by subjecting pupae to LD 12:12 cycles in which the light intensity of the photophase was 500 lux in all LD cycles, while that of the scotophase was either 0 lux (complete darkness, DD), 0.5, 5, or 50 lux. In the 0 lux LAN condition (i.e., the control experiment), the eclosion peak was approximately 2 h after lights-on, and the 24 h eclosion rhythm was not as strong as in the 0.5 lux LAN condition. The entrainment pattern in 0.5 lux LAN was strikingly similar to that in the field, as the 0.5 lux LAN condition is comparable to the full moonlight intensity in the tropics. LAN at 0.5 lux dramatically altered both parameters of entrainment, as the eclosion peak was advanced by approximately 4 h and the 24 h eclosion rhythm was better than that of the control experiment. LAN at 5 lux, however, resulted in a weak eclosion rhythm that peaked in the subjective forenoon. Interestingly, the 50 lux LAN condition rendered the eclosion events unambiguously arrhythmic. After-effects of LAN on the period (tau) of the free-running rhythm and the nature of eclosion rhythm were also determined in DD by a single LD 12:12 to DD transfer. After-effects of the LAN intensity were observed on both the tau and nature of the eclosion rhythm in all four experiments. Pupae raised in 0.5 lux LAN exhibited the shortest tau (20.6 +/- 0.2 h, N = 11 for this and subsequent values) and the most robust rhythm, while pupae raised in 50 lux LAN had the longest tau (29.5 +/- 0.2 h) and weakest rhythm in DD. Thus, these results demonstrate the intensity of LAN, varying from 0 to 50 lux, profoundly influences the parameters of entrainment as well as free-running rhythmicity of D. jambulina. Moreover, the observed arrhythmicity in LD 12:12 cycles caused by the 50 lux LAN condition appeared to be the masking effect of relatively bright light at night, as the LD 12:12 to DD transfer restored the rhythmicity, although it was rather weak.
