Plasma melatonin levels in Japanese quail exposed to dim light are determined by subjective interpretation of day and night, not light intensity.

Plasma melatonin levels were measured in male Japanese quail exposed to lighting schedules consisting of combinations of bright light (2000 or 1500 lx), darkness, or dim light (2 lx) or to constant dim light. Melatonin levels in dim light were dependent upon the relative intensity of accompanying phases, being significantly higher when dim light was subjective night than when it was subjective day. There was no significant melatonin rhythm in constant dim light, even on the first day of constant dim light exposure. Melatonin levels were intermediate when dim light was accompanied by both bright light and darkness. These results indicate that melatonin secretion in birds does not depend solely on light intensity. Furthermore, these results suggest that the avian circadian system may be more sensitive to environmental cues than its mammalian counterpart.