  Drosophila's circadian clock can be perturbed by magnetic fields, as well as
by lithium administration. Cryptochromes are critical for the circadian clock.
Further, the radical pairs in cryptochrome also can explain magnetoreception in
animals. Based on a simple radical pair mechanism model of the animal magnetic
compass, we show that both magnetic fields and lithium can influence the spin
dynamics of the naturally occurring radical pairs and hence modulate the
circadian clock's rhythms. Using a simple chemical oscillator model for the
circadian clock, we show that the spin dynamics influence a rate in the
chemical oscillator model, which translates into a change in the circadian
period. Our model can reproduce the results of two independent experiments,
magnetic fields and lithium effects on the circadian clock. Our model predicts
that stronger magnetic fields would shorten the clock's period. We also predict
that lithium influences the clock in an isotope-dependent manner. Furthermore,
our model also predicts that magnetic fields and hyperfine interactions
modulate oxidative stress. The findings of this work suggest that quantum
nature and entanglement of radical pairs might play roles in the brain, as
another piece of evidence in addition to recent results on xenon anesthesia and
lithium effects on hyperactivity.
