Changes in phase-angle under light-dark cycles influenced by nonphotic stimulation.

Most work looking at nonphotic effects on circadian rhythms is conducted when animals are held under freerunning conditions, usually constant darkness. However, for nonphotic effects to be functionally significant, they should be demonstrable under conditions in which most animals live, i.e., a 24-hr light-dark cycle (LD). Syrian hamsters held in LD 6:18 were administered nonphotic stimulation in the form of a 3-hr confinement to a novel wheel starting about 6 hr before the start of their normal nightly activity bout. This resulted in a 2.5-hr advance of their activity rhythm on the next day that gradually receded to about 1.5 hr over the next 10 days. When hamsters held in LD 6:18 were given five novel wheel confinements over 13 days their nightly activity onset advanced 3 hr and remained at that phase for at least 2 weeks. Home cage wheel deprivation experiments indicated that high levels of home cage activity are necessary to maintain the advanced phase. These results show that nonphotic stimulation can have large, long-lasting effects on daily rhythms in LD and suggest a possible mechanism whereby nocturnal rodents might achieve phase flexibility in response to seasonal changes.