DEPENDENCE OF THE VARIATION IN ALERTNESS ON THE PINEAL-GLAND

There is a diurnal variation in the pineal serotonin content, with a m aximum at 7-8 h after the onset of light and a minimum at 3-4 h after the onset of darkness in rats. The levels of serotonin and melatonin c an rapidly be altered by changing the level of ambient light. The diur nal changes in serotonergic activity are hypothesized to counteract tw o phenomena. First, the increased levels of serotonin would decrease t he high propensity to drowsiness in the afternoon. Second, the decreas ed levels of serotonin would also decrease the high propensity to waki ng up at night, which occurs because of decreased melatonin secretion.