Light intensity and splitting in the golden hamster.

Hamster circadian activity rhythms split into two components during prolonged exposure to conditions of constant light (LL). Several aspects of this phenomenon were examined in this study. The frequency of splitting was significantly greater among animals exposed to LL of 100 lux intensity (LL100) compared with animals in LL10. Animals that split had significantly longer free-running periods (tau) compared to nonsplitters and the decrease in tau associated with splitting was highly correlated with the presplit tau. Splitting was also observed under continuous dim light which fluctuated rhythmically from 5-10 lux. Thus, splitting of the circadian rhythm of activity is positively correlated with LL intensity with an LL intensity threshold for the induction of splitting in the range of 3-5 lux.