Summary This project investigated the relationship between exposure to light and 24-h patterns of sleep and crying in young, healthy, full-term babies living at home and following a normal domestic routine. Measures included an ankle worn activity monitor, an external light monitor and the Barr Baby Day Diary in which parents recorded periods of sleep, crying, feeding and other behaviours at 5-min intervals throughout the 24-h period. Fifty-six babies (26 males and 30 females) were monitored across three consecutive days at 6, 9 and 12 weeks of age. There was an early evening peak in crying which was associated with reduced sleep at 6 weeks. Across the trials there was a gradual shift towards a greater proportion of sleep occurring at night. Sleeping well at 6 weeks was a good indication of more night-time sleep at 9 and 12 weeks. Babies who slept well at night were exposed to significantly more light in the early afternoon period. These data suggest that light in the normal domestic setting influences the development of the circadian system.