The adjustment of the melatonin rhythm of rats by 90-min dark pulses.

The pineal melatonin patterns were measured in rats kept 1, 2, 3, 4, 6, 8, 16, or 28 days under the lighting conditions with a dark period of 90 min only (from 18:00 to 19:30 h). The samples were collected at 30-min intervals, and melatonin was measured by radioimmunoassay. During the first and second dark pulse the melatonin levels were low, but thereafter they were significantly higher than the usual daytime levels. The increase of the synthesis started within 15 min after the light offset. The results suggest a stable adjustment of the melatonin rhythm by the dark pulses.