The nocturnal activity of fruit flies exposed to artificial moonlight is partly caused by direct light effects on the activity level that bypass the endogenous clock

Chronobiol Int. 2009 Feb;26(2):151-66. doi: 10.1080/07420520902747124.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • ARNTL Transcription Factors
  • Animals
  • Basic Helix-Loop-Helix Transcription Factors / genetics
  • Basic Helix-Loop-Helix Transcription Factors / metabolism
  • Behavior, Animal / physiology
  • Biological Clocks / physiology*
  • CLOCK Proteins
  • Circadian Rhythm / physiology*
  • Darkness
  • Drosophila Proteins / genetics
  • Drosophila Proteins / metabolism
  • Drosophila melanogaster / physiology*
  • Light*
  • Moon*
  • Motor Activity / physiology*
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism
  • Period Circadian Proteins
  • Photoperiod
  • Transcription Factors / genetics
  • Transcription Factors / metabolism


  • ARNTL Transcription Factors
  • Basic Helix-Loop-Helix Transcription Factors
  • Clk protein, Drosophila
  • Drosophila Proteins
  • Nuclear Proteins
  • PER protein, Drosophila
  • Period Circadian Proteins
  • Transcription Factors
  • cyc protein, Drosophila
  • tim protein, Drosophila
  • CLOCK Proteins