Ultradian and circadian rhythms of sleep-wake and food-intake behavior during early infancy

The early development of sleep-wake and food-intake rhythms in human infant s is reviewed. The development of a 24h day-night rhythm contains two obser vable developmental processes: the alterations in the periodic structure of behavior (decreased ultradian, increased circadian components) and the pro cess of synchronization to external time (entrainment). The authors present the results of their studies involving 26 German children and compare them with previous investigations. In their research, it became evident that, d uring the first weeks of life, the time pattern of sleep-wake and food-inta ke behavior is characterized by different ultradian periodicities, ranging from 2h to 8h. In the course of further ontogenesis, the share of ultradian rhythms in the sleep-wake behavior decreases, while it remains dominant fo r food-intake behavior The circadian component is established as early as t he first weeks of life and increases in the months that follow. Besides, th e authors' study supports the notion of broad interindividual variation in ultradian rhythms and in the development of a day-night rhythm. Examples of free-running rhythms of sleep-wake and food-intake behavior by various aut hors are strong indicators of the endogenous nature of the circadian rhythm s in infants and show that the internal clock is already functioning at bir th. It is still uncertain when the process of synchronization to external a nd social time cues begins and how differences in the maturation of percept ive organs affect the importance of time cues for the entrainment. Preparta lly, the physiological maternal entrainment factors and mother-fetus intera ctions may be most important; during the first weeks of Life, the social ti me cues gain importance, while light acts as a dominant "zeitgeber" at a la ter time only.