Sleep rhythmicity in infants: index of stress or maturation

The rhythmicity of bouts of quiet sleep (QS) was assessed, starting immediately after the baby's birth. The subjects were 58 healthy fullterm, single-birth, newborn infants, 26 females and 32 males. Using a non-intrusive recording procedure, their sleep was monitored for 24-h periods on the 1st and 2nd postnatal days in the hospital, then for 2 days in the home at 6 months. The cyclicity index permitted determination of the degree of periodicity as well as whether the recurrence of QS bouts showed significant periodicity. The number of subjects with significant cyclicity increased from 34% of the group on postnatal day 1 to 73% at 6 months; cyclicity scores (CS) increased from 0.71 to 0.86; and mean cycle length increased from 51 to 57 min. Infants with significant cyclicity on day 1 had lower mental scores at 6 months; but infants with significant cyclicity at 6 months had higher mental scores at 1 year. In addition, the infants with significant cyclicity on day 1 had lower birth weights and were born to younger mothers; but these relationships were also reversed at 6 months. Finally, cyclicity scores at 6 months were significantly correlated with 1-year mental scores, but the function of this relationship was quadratic. Thus, while significant cyclicity was found from the first postnatal day, the results suggest that regularity in QS cycles in the newborn period has negative implications for development, while such regularity at 6 months has positive implications-although excessive rigidity in rhythms at the later age, in terms of extremely high cyclicity scores, was also an indicator of developmental compromise.