Development of human sleep and wakefulness rhythm during the first six months of life: Discontinuous changes at the 7th and 12th week after birth

The development of the human circadian rhythm of sleep and wakefulness during the first 6 months of life was investigated in ten (7 male and 3 female) full-term infants. The mothers were asked to fill out the sleep logs of their babies over the six months after birth. On the basis of the sleep logs, the authors scored each 15 min unit of sleep as 'one' and that of wakefulness as 'zero.' The strings of data were analyzed by the autocorrelation method to investigate the development of the circadian rhythmicity of each infant's sleep during each week. Then the authors classified the patterns of correlogram from the 2nd to the 26th week using cluster analysis. The analysis classified these patterns into three groups. The first group (pattern A) showed scarce evidence of a circadian component. The second one (pattern B) had a prominent circadian component. The last one (pattern C) was characterized by its prominent circadian positive peak and negative peak around 12 hrs periodicity, which represent very scarce amount of daytime sleep. Almost all the infants changed their patterns from A to B around the 7th week, then some of the infants changed their patterns from B to C at the 12th week. The first discontinuous change in the sleep and wakefulness circadian rhythm around the 7th week coincides with the period of an important reorganization in sensorimotor behavior, which many other studies suggest. The second discontinuous change might be considered as the first sign of individual differences concerning daytime naps or daytime sleepiness.