Reorganisation of circadian activity and the pacemaker circuit under novel light regimes

Many environmental features are cyclic, with predictable changes across
the day, seasons and latitudes. Additionally, anthropogenic, artificial
light-induced changes in photoperiod or shiftwork-driven novel light/dark
cycles also occur. Endogenous timekeepers or circadian clocks help
organisms cope with such changes. The remarkable plasticity of clocks is
evident in the waveforms of behavioural and molecular rhythms they govern.
Despite detailed mechanistic insights into the functioning of the
circadian clock, practical means to manipulate activity waveform are
lacking. Previous studies using a nocturnal rodent model showed that novel
light regimes caused locomotor activity to bifurcate such that mice showed
two bouts of activity restricted to the dimly lit phases. Here, we explore
the generalizability of these findings and leverage the genetic toolkit of
Drosophila melanogaster to obtain mechanistic insights into this unique
phenomenon. We find that dim scotopic illumination of specific durations
induces circadian photoreceptor CRYPTOCHROME-dependent activity
bifurcation in male flies. We show circadian re-organisation of the
pacemaker circuit wherein the ‘evening’ neurons regulate the timing of
both bouts of activity under novel light regimes. Our findings indicate
that environmental regimes can be exploited to design light regimes to
ease the circadian waveform into synchronising with challenging
conditions.