Emerging and entraining patterns of the sleep–wake rhythm in preterm and term infants

It has been repeatedly reported that the sleep–wake rhythm in infants entrains around 3–4 months of age after a transient free-run rhythm. To clarify the emerging and entraining patterns of the sleep–wake rhythm, the sleep and wakefulness of 84 infants (44 preterm and 40 term infants) were longitudinally recorded at home for more than 16 weeks by the day-by-day plot method. Our results showed that the entrained sleep–wake rhythm emerged after transient manifestation of either ultradian or irregular sleep–wake patterns for 3–4 weeks in 75% of the infants. Only 7% of the infants showed a free-running sleep–wake rhythm before the entrainment. These facts suggest that most infants would be entrained to an ordinary daily schedule of mothers without expression of overt free-running rhythm of the biological clock. The mean age of the entrainment was 44.8 postconceptional weeks. There were no significant differences in either frequency of each pattern or the mean age of the entrainment, between preterm and term infants. In conclusion, the entrained sleep–wake rhythm emerges around 1 corrected month, after ultradian patterns in the majority of infants.