Earth's magnetic field can influence animals' circadian clocks, surprisingly enough through the photoreceptor cryptochrome, which is activated by blue light.
Because this photoreceptor is known to occur in the fruitfly Drosophila, Charlotte Helfrich-Förster of the University of Regensburg in Germany and her colleagues subjected flies to applied magnetic fields under both red and blue light. The static magnetic fields slowed the flies' circadian clocks, but only in the presence of blue light. Flies under red light showed no response. Mutant flies overexpressing cryptochrome had an even more pronounced response.
Thus the authors suggest the fly could be a model system for cryptochrome-dependent magnetic sensitivity — which may be used, for example, by migratory birds for compass orientation.