The development of circadian rhythms in a human infant

The development of circadian rhythms in a human infant Study Objectives-Thi s study examines the ontogeny and interaction of circadian rhythms of sleep , wake, temperature, melatonin, and feeding in the human newborn, and the i nfluence of photic and non-photic factors on the initiation of entrainment. Design-An infant's sleep-wake state, temperature, and salivary melatonin w ere monitored from birth to 6 months. Temperature was obtained every hour, and the infant's sleep/eating onset/termination were observed continuously and recorded until day 182. Salivary melatonin was obtained weekly for a 24 -hour period, starting at week 3. Setting-The infant slept in his parents' bedroom. All household members awakened, retired, and ate meals according t o a fixed schedule during the study, while the infant ate, slept, and woke on demand. Participants-A healthy male infant was the subject. Biological p arents gathered data continuously for six months. Interventions-The infant' s schedule was on-demand; the household's was fixed. Illumination was restr icted to sunlight. Measurements and Results-The circadian rhythm of tempera ture appeared first, soon after birth, and became statistically significant within one week. The wake circadian rhythm appeared second, attaining sign ificance at day 45; approximately the same time that increased melatonin co ncentration began to occur at sunset. The sleep circadian rhythm appeared l ast, attaining significance after day 56. Ninety to 120 minute zones of sus tained wakefulness first appeared in the second month of life subsequent to awakening and prior to sleep onset. The infant's nocturnal sleep-onset was coupled to sunset before day 60 and subsequently to family bedtime, giving evidence of initial photic entrainment followed by social entrainment. Con clusions-Circadian rhythms appeared much more rapidly in this infant than p reviously reported; their rapid appearance was probably facilitated by maxi mal exposure to sunlight, and regular social cues. These lighting condition s replicate universal infant experience prior to the invention of artificia l light.