Twenty healthy preterm babies with gestational ages between 313/7 and 353/7 weeks were studied in a low-risk neonatal care unit. Sleep-wake cycle data were collected by the nursery staff through behavioral observation every 10 min as well as the spontaneous or induced nature of awakenings. The data were divided into 10-min epochs, grouped in a series of seven consecutive days, and submitted to spectral analysis. Awakenings were analyzed through a method which calculates the duration of the intervals between the spontaneous or induced awakenings. Oral temperature data, collected every four hours (three consecutive days) were analyzed with the Cosinor technique. An average of 26% of the sleep behavior can be explained by rhythmicity, which is composed by several ultradian frequencies. Three-hour oscillation was the most potent component of the spectrum. Six children expressed a 24-h rhythm. Spontaneous awakenings showed several ultradian rhythms of low potency, whereas induced awakenings expressed a more potent overall rhythmicity. Only one baby expressed a temperature rhythm in the circadian range during the second week of life. These results support the idea that at least part of the sleep-wake rhythmicity identified in babies maintained in a neonatal care unit is generated by external influences.