Cycles of dim red light capable of entraining circadian rhythms of rats after long‐term exposure to constant white light

Abstract

Abstract In circadian studies dim red light is often used as a convenient alternative for darkness because it would not affect the pacemaking system. In this study male Wistar rats were first exposed to constant light (LL) for 3 months and next to LD 14:10 cycles of dim red light (> 610 nm; 1 ‐ 5 μWatt/cm2) while recording feeding behaviour. All rats displayed entrainment of the circadian feeding rhythm to the red light cycle. A phase reversal of the LD cycle was followed by a gradual reentrainment over 5–8 weeks via delaying transients. It was concluded that dim red light acts as a weak Zeitgeber and that cone‐like photoreceptor surviving long‐term LL exposure could mediate entrainment.