Dependence of the variation in alertness on the pineal gland.

There is a diurnal variation in the pineal serotonin content, with a maximum 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 can rapidly be altered by changing the level of ambient light. The diurnal changes in serotonergic activity are hypothesized to counteract two phenomena. First, the increased levels of serotonin would decrease the high propensity to drowsiness in the afternoon. Second, the decreased levels of serotonin would also decrease the high propensity to waking up at night, which occurs because of decreased melatonin secretion.