When an animal is exposed to constant, intense light, the internal clock goes haywire, losing all sense of night and day.
Fruit flies exhibit the same reaction, and humans are predicted to respond similarly.
In the laboratories of Howard Hughes Medical Institute investigator Michael Rosbash and Jeffrey Hall at Brandeis University, however, there is a strain of mutant flies that maintains a steady clock when barraged with intense light. The flies carry a crippled light-reactive pigment called a cryptochrome. However, we found that these cry b mutants did not show such arrhythmia under constant light, as measured by their activity. If there were circadian photoreceptors other than dCRY, then constant light should have produced such arrhythmia. The cry b mutant flies still present a significant mystery, Rosbash says, because they show the normal daily rhythmic cycle of light-dark activity, even with the presumably crippled photoreceptor molecules.