Circadian rhythm of self-exposure to light in the golden hamster

Golden hamsters were maintained in constant darkness with free access to a running wheel and to a lever that they could press to turn on the lights. The animals exhibited a circadian rhythm of lever pressing (and, therefore, of self-exposure to light) in synchrony with the rhythm of running-wheel activity. Although most light exposure occurred during early subjective night (when single light pulses are known to cause phase delays of circadian rhythms), there was no significant difference between the circadian periods before and after the animals were allowed to self-stimulate. It is suggested that the conservation of circadian period was due to a reduction in photic sensitivity caused by the observed pattern of light exposure (multiple brief pulses rather than single long pulses).