nocturnal animals have higher body temperatures, greater activity, rising serotonin, and diminishing cortisol during the night—the inverse of diurnal animals. nocturnal and diurnal animals "both" have increased electrical activity in the suprachiasmatic nucleus, and corresponding secretion of melatonin from the pineal gland, at night. nocturnal mammals, which tend to stay awake at night, have higher melatonin at night just like diurnal mammals do. and, although removing the pineal gland in many animals abolishes melatonin rhythms, it does not stop circadian rhythms altogether—though it may alter them and weaken their responsiveness to light cues. cortisol levels in diurnal animals typically rise throughout the night, peak in the awakening hours, and diminish during the day. in diurnal animals, sleepiness increases during the night.