Colic, sleep inertia, melatonin and circannual rhythms.
Colic is periodic behavior occurring at the end of the day during the first 3 months of life characterized by crying. It is hypothesized that the crying at the end of the day is due to sleep inertia or a state dissociation during which the infant is simultaneously partially awake and partially asleep because of the absence of a melatonin diurnal rhythm. The melatonin timing mechanisms, which codes for day length, is initiated prenatally by the maternal pineal gland, and after 3 months postnatally, the melatonin nocturnal secretion rhythm is maintained by the infant's pineal gland. To record the seasonal variation in day length away from the equator, 12 months are required to complete the melatonin chemical calendar. This circannual process is only 9 months completed at the time of birth, and 3 additional months are needed during which the infant may have difficulty reconciling cues for the timing of evening sleep due to discrepancies between the expected photoperiod derived from the prenatal maternal pineal melatonin circadian rhythm and the postnatally experienced photoperiod. Data is presented showing that the incidence of colic increases with increasing latitude in support of the hypothesis that infant crying at the end of the day during the first 3 months represents the last quarter of a circannual photoperiodic development process.