Scheduled wheel access during daytime: A method for studying conflicting zeitgebers.

It is often stated that light is the primary environmental cue (zeitgeber) for entrainment of circadian clocks. Here, we use a new conflict test design in Syrian hamsters comparing the strength of a photic zeitgeber to that of a non-photic cue, i.e. wheel availability. Re-entrainment to an inverted LD cycle was significantly slowed down in the nocturnal hamster by restricting wheel access to the light phase of the inverted LD cycle. This effect is more pronounced if the illuminance level of the entraining lights is 0.1 lx compared to 6 lx. In this conflict design, the hamsters did not re-entrain to an inverted LD cycle for up to four weeks (when the experiment ended), but voluntarily ran during the light phase. This approximates the situation in people subjected to shift work or jet lag.