The sleep states and the regularity of quiet sleep (QS) respiration were investigated in premature infants who were provided a "breathing" teddy bear. The bear (BrBr) is a source of optional rhythmic stimulation that reflects the breathing rate of the individual infant it is with. At 33 weeks CA, 19 premature infants were given a BrBr and 17 were given a nonbreathing bear (N-BrBr). At 35 weeks CA, and again at 45 weeks CA, a 1-2-h interfeed motility recording was obtained. These analog signals were scored for active sleep, QS, and wakefulness; and each 10-s epoch of QS was judged for regularity of respiration using a four-point rating scale. At 35 weeks, the BrBr babies showed slower and more regular respiration during QS. At 45 weeks, the BrBr babies showed more QS and less active sleep. At both ages, only the BrBr babies showed a correlation between respiratory regulatory and the amount of QS. The findings suggest facilitation of neurobehavioral development as well as entrainment from optional stimulation, which reflects one of the infant's own biological rhythms.