How can light/dark cycles reset circadian clocks?

Circadian clocks are self-sustained molecular oscillators and keep ticking in the absence of any environmental cues. But in order to be a useful tool for advantageous timing of biological processes in a natural day/night setting, these environmental changes must be able to influence (or reset) the molecular oscillations.

In flies, an important photoreceptor mediating this ‘light-synchronization’ is the blue-light photoreceptor Cryptochrome (Cry) (Stanewsky et al., 1998). Interestingly, Cry is expressed within the brain clock neurons that drive the rest/activity cycles of the fly—so it’s a photoreceptor expressed in the brain. But it is clear that the visual photoreceptors in the retina of the fly are also important for light-synchronization, as well as sub-retinal structure known as the Hofbauer-Buchner eyelet (Figure) (Yoshii et al., 2009, Helfrich-Foerster et al., 2001, Stanewsky et al., 1998, Veleri et al., 2007, Emery et al., 2000). Current research in our lab tries to determine why such a complexity of photoreceptors and light-inputs is needed and used for proper clock resetting, and which role the non-visual Hofbauer-Buchner eyelet may play.