The aims of this study were: (1) to investigate the evolution of the sleep pattern in preterm newborns during their first month of life; (2) to assess the influence of light-dark on the sleep pattern; and (3) to compare this pattern with that of full-term newborns. The population consisted of 60 healthy, preterm newborns and 63 full-term newborns, divided into four age groups, 1 week apart, throughout the first month of life. Preterm newborns were further divided into five groups according to conceptional (corrected) age. An observer took note every 30 min, for 24 h, of sleep or wakefulness in every case. The average sleeping time in preterm groups according to postnatal age remained unchanged during the first month of life: 17.57 h on day 1 and 17.15 h on day 28. When the preterm infants were re-grouped according to conceptional age, average daily sleep was 17.86 h at 32 weeks and 15.22 h at 37 weeks. The full-term newborns had an average daily sleep of 14.78 h on day 1 and 11.94 h on day 28, with a decrease throughout week 4 of life (p < 0.001). The decrease in daily sleeping time in the full-term groups, took place at the expense of the daylight span, where there was a decrease throughout the first month of life (p < 0.01). There were no differences in preterm newborns during the light and dark phases. A progressive synchronization of sleep to the light-dark was seen in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)