Relationship between light intensity and the melatonin and drinking rhythms of rats

We examined the effect of varying ambient light intensity on the amplitudes of the daily rhythms in urinary melatonin excreted or water consumed per day. Animals were initially exposed to one of two types of diurnal regimens (dim light: darkness or bright light: darkness) designed to simulate their lighting situations in nature or in the laboratory. They were then placed under continuous light, at one of four intensities: total darkness, very dim light (0.005-0.01 microW/cm2), dim light (0.1-0.3 microW/cm2) and bright light (45-110 microW/cm2). Under continuous lighting the amplitudes of melatonin rhythms and the concentrations of melatonin excreted per 24-hour period decreased as the intensity of illumination was increased. A 50% decrease in the rhythm's amplitude, compared with the amplitude observed when the same rats had been under a diurnal lighting regimen, required irradiances of about 0.05 microW/cm2, which is on the same order as the intensity provided by full moonlight (0.20 microW/cm2). Water consumption rhythms were less altered by continuous light: only the brightest irradiances (45-110 microW/cm2) significantly reduced amplitude by 40-60%.