Circadian light input in plants, flies and mammals.

The rotation of our planet results in daily changes in light and darkness, as well as seasons with characteristic photoperiods. Adaptation to these daily and seasonal changes in light properties (and associated changes in the environment) is important to the sustained survival of higher life forms on our planet. Many organisms use their intrinsic circadian oscillator or clock to orchestrate daily rhythms in behaviour and physiology to adapt to diurnal changes. Some higher organisms use the same oscillator to monitor day length in selecting the appropriate season for reproductive behaviour. Organisms have developed irradiance measurement mechanisms to ignore photic noise (lightning, moonlight), and use the light of dusk and dawn for circadian photoentrainment. They have also devised multiple photoreceptors and signalling cascades to buffer against changes in the spectral composition of natural light. The interaction of the clock with ambient light is, therefore, quite intricate.