Infants were brought to the laboratory adjacent to the nursery at approximately 07:30 h at which time they had EKG leads attached. They were then placed in a radiantly warmed, clear plastic incubator and maintained under thermoneutral conditions. No physical constraints such as swaddling were employed during the study period. Studies began following 08:00 h feeding and continued until the 14:00 h feeding. The studies were interrupted for 11:00 h feeding. Otherwise, the infants were left undisturbed during the inter-feed intervals. The volume and composition of the two feeds were identical. Sleep state coding began 10 min after the 08:00 h feeding, continued until 11:00 h feeding, resumed after the 11:00 h feeding and terminated prior to the 14:00 h feeding. Behavior codes were assigned by direct observation each minute using a scoring system developed and validated in our laboratory [14]. Briefly, active sleep was coded whenever at least one rapid eye movement was observed during the minute. In addition to small body movements typical of active sleep, movements of whole extremities and the torso were seen in this state. Stretching, yawning, whimpering, sucking and grimacing were also present occasionally. Quiet sleep was designated when the infant was asleep without rapid eye movements. During quiet sleep the infant was relaxed and appeared `rag-doll' floppy; movements were limited to startles and non-nutritive sucking or jaw jerks. Indeterminate state was coded when small body movements were observed, without rapid eye movements. Codes were also assigned for wakefulness, crying, and feeding. Behavioral state of LBW infants as assigned in this study is known to be highly correlated with electroencephalographically coded sleep state [15]. Bedside coding of behavioral state in the low birth weight infant is more difficult than in the term infant as activity cycles are less distinct and correlations among state variables are less close. The use of 1-min epochs without the use of smoothing algorithms leads to more minute to minute variability in state assignment but has the advantage of maintaining a tighter relationship between the assigned state and the simultaneous changes in physiologic variables.