Links between time of day, sleep and unhappy mood in early infancy: An intensive case study

Crying in early infancy is usually worse in the evening, peaks about six we eks after birth, and then gradually decreases, but it is not known why. Thi s study tests the idea that the pattern of affect in early infancy is relat ed to the development of circadian control over the sleep-wake cycle. Sleep and unhappy mood measures for a female infant were recorded in a diary by her parents for 210 consecutive days, starting a fortnight after her birth. Analysis of variance and cosinor analysis of the data revealed a circadian rhythm in unhappy mood that peaked in the evening, was present irrespectiv e of food intake, and declined in strength after a few months. Developments in the diurnal pattern of unhappy mood were paralleled by increasing circa dian rhythmicity in time of falling asleep and sleep duration. Regression a nalysis showed that unhappy mood was influenced by an interaction between t ime of sleep and duration of sleep. The results were in accord with recent research on mood in adults and suggest that the pattern of affect in early infancy may occur because the sleep-wake cycle is not yet aligned with the circadian timing system.