Development of the 24-h rest-activity pattern in human infants.

The development of the 24-h rest-activity pattern was investigated in human infants under naturalistic conditions as assessed by continuous actigraphy. Seven infants and their mothers were recorded for 4 (n=1), 6 (n=5) and 12 months (n=1) after birth. Periodogram analysis of rest-activity data was performed over consecutive 10-day intervals. A weak 24-h rest-activity pattern was already present in some infants during the newborn period. The magnitude of the 24-h component in individual periodograms increased across the first months following a saturating function. The time constants of fitted saturating exponential functions--reflecting the rate of development of the 24-h pattern--ranged from 49 to 110 days (n=6) indicating a large interindividual variability. Furthermore, intraindividual variation was observed; the magnitude of the 24-h rest-activity component showed fluctuations around the trend. Miniaturized actigraphs are ideal tools for long-term longitudinal monitoring of rest-activity behavior in infants.