Abstract

Studies have shown that light pollution impairs biorhythms through alteration in expressions of circadian clocks. The duration related effect of photoperiodic perturbation on pattern of endogenous rhythms is not clear. 20 adult female rats displaying 4-5 days estrous cycle length and weighing 148-154 g were randomly divided into four groups of five animals each. While groups A and B were maintained under natural 12 h (light/ dark) cycle (3-20lux) for 1 and 8 weeks respectively, animals in groups C and D were maintained under alternate schedule of 20h light/4h dark cycle (120-150 lux) for 1 and 8 weeks respectively. Body temperature rhythm, estrous cycle pattern and nocturnal plasma melatonin were determined. The results indicated dysrhythmia in body temperature evidenced by the significant change (P<0.05) in nighttime and day time temperatures. There was a significant increase (P<0.05) in estrous cycle length and estrous cycle ratio. There was also a significant decrease (P<0.05) in nocturnal melatonin secretion. While the effect of photoperiodic perturbation on melatonin secretion and estrous cycle ratio was duration dependent, the effect on nighttime and daytime body temperature, estrous cycle length was not duration dependent. We concluded that photoperiodic perturbation caused no sustainable effect on estrous cycle ratio and nocturnal plasma melatonin secretion.