Developmental changes in the sleep–wake rhythm of preterm infants were compared with those of full-term infants, to clarify the timing of the developmental change of the sleep–wake rhythm and its dependence on either conception or birth timing. We obtained sleep log data for two preterm infants, and compared these with previous data gathered from 10 full-term infants over a period from 2 weeks after birth to 3 months after their expected delivery dates. Infant sleep logs were analyzed with the autocorrelation method to investigate the development of the circadian rhythm in each infant's sleep patterns during each weekly session. We then classified the patterns of correlograms into three groups using cluster analysis. The first group (pattern A) showed little evidence of any circadian component. The second (pattern B) had a prominent circadian component. The last (pattern C) was characterized by its prominent circadian peak and a negative peak in about a 12-h cycle, which represented very little daytime sleep. There were no differences in the sleep-wakefulness patterns of full-term infants and preterm infants when we compared their sleep log data using postmenstrual age. Almost all of the infants in both groups changed their patterns from A to B around the 46th week. Considering these findings, these changes in the circadian rhythm of sleep and wakefulness were viewed as showing dependence on conception rather than on birth timing.