A SHORT RED-LIGHT PULSE DURING DARK PHASE OF LD-CYCLE PERTURBS THE HAMSTERS CIRCADIAN CLOCK

In this study we investigated the influence of red Light, which natura lly occurs during dawn and dusk, on locomotor activity and body temper ature rhythms of Djungarian hamsters (Phonopus sungarus). A single wea k red light pulse given 2 h before regular lights on had acute as well as long-term effects persisting for several days following exposure. The hamsters immediately stopped their locomotor activity, accompanied by a drop in body temperature. In the following undisturbed nights (L D 16:8) the nocturnal activity stopped earlier than usual. This lastin g effect of the light pulse was more pronounced than the acute effect. The activity phase compressed gradually during 3 to 5 days after the light pulse was administered while time of activity onset was almost u naffected. It took 6 to 11 days for complete recovery of the original activity phase. The maximal activity compression and the recovery peri od depended on the duration of the single red light pulse and its inte nsity. Red light pulses of 15 min duration were about twice effective as 1 min pulses; and the effect of a red light pulse of 130 mW/m(2) wa s about 1.5 times stronger than a 30 mW/m(2) red light pulse. The maxi mal value of activity phase compression reached in this experiment was 2.5 + 0.2 h with a recovery period of 11.1 +/- 0.3 days following a g iven red light pulse of 90 mW/m(2) and 15 min. The ''morning oscillato r'' seems to be persistently affected. This indicates a very high phot osensitivity of the Djungarian hamster's circadian system to red light .