The purpose of the current study was to investigate the development of sleep-wake and melatonin diurnal rhythms over the first 3 months of life, and the potential effect of bed-sharing on their development. It was hypothesized that increased maternal contact through bed-sharing would affect the development of rhythms in human infants. Ten solitary-sleeping and 8 bed-sharing infants' sleep-wake patterns and melatonin secretion were examined for 72 h at 1 and 3 months of age in their homes. Infants wore actigraphs on their ankles to study sleep-wake patterns. 6-Sulphatoxymelatonin was obtained through urine extracted from each diaper used over the 72-h study period. No significant differences were apparent in the timing of appearance or magnitude of sleep-wake or melatonin rhythms between bed-sharing and solitary-sleeping infants. Sleep-wake results were in the expected direction, with bed-sharing infants displaying more robust rhythms. A large degree of individual variability was evident in both rhythms, especially at 1 month. Three infants' parents regularly used a bright light source at night for feedings and diaper changes; the rhythms of these infants were less robust than the rest of the sample. Trends were mostly in the hypothesized direction and deserve attempts at replication with a larger sample. (Contains 2 figures, 4 tables and 7 notes.)