Sleep rhythmicity in premature infants: Implications for developmental status

Abstract

Ultradian and diurnal rhythms in premature infants were investigated by assessing cyclicity of quiet sleep (QS) and the diurnal distribution of this cyclicity. The sleep of 49 preterm infants was recorded in the hospital for three successive 24-hour periods at 36 weeks conceptional age (CA), and 42 of the infants were recorded in the home for two 24-hour periods when they were 6 months old. Sleep was recorded nonintrusively by means of the motility monitoring system, which does not require instrumentation of the subject. Cyclicity was assessed using a procedure that permits assessment of significance as well as degree of cyclicity. Twenty of the 49 infants at the preterm age and 37 of the 42 infants at 6 months had sleep episodes with significant cyclicity. Mean cyclicity scores increased from 0.61 to 0.81 over age, but the cycle length of approximately 60 minutes did not change. There was no evidence for individual consistency across the two ages in any of the sleep or cyclicity measures. Evidence for diurnal differences was present from the preterm period. At both ages, there were far more analyzable sleep episodes and higher cyclicity at night. At the preterm period, cyclicity measures were negatively related to indices of advanced perinatal status as well as 6-month mental scores; at 6 months, the cyclicity measures were positively related to perinatal measures as well as mental scores. These results indicate the necessity for different interpretations of periodicity at the preterm and later age. When the QS and active sleep (AS) components of the sleep cycle were examined, QS was not found to be related to mental scores at either age; whereas, the AS component was related to mental scores at both ages-but negatively at the preterm age and positively at 6 months. The results indicate that AS, in this context, is an indicator of more general neurobehavioral competence, but in reverse directions at the two ages. These results suggest that different regulatory processes are controlling the temporal sequencing of slates over the developmental course. It is proposed that preterm cyclicity is a response to ongoing stress in the preterm infants, while cyclicity at 6 months reflects more directly central neural competence.