Objective Infants who cry a lot, or are unsettled in the night, are common sources of concern for parents and costly problems for health services. The two types of problems have been linked together and attributed to a general disturbance of infant regulation. Yet the infant behaviours involved present differently, at separate ages and times of day. To clarify causation, this study aims to assess whether prolonged crying at 5–6 weeks (the peak age for crying) predicts which infants are unsettled in the night at 12 weeks of age (when most infants become settled at night).

Methods Data from two longitudinal studies are analysed. Infant crying data were obtained from validated behaviour diaries; sleep-waking data from standard parental questionnaires.

Results A significant, weak relationship was found between crying at 5–6 weeks and 12-week night waking and signalling in one study, but not the other. Most infants who met the definition for prolonged crying/colic at 5–6 weeks were settled during the night at 12 weeks of age; they were not more likely than other infants to be unsettled.

Conclusions Most infants who cry a lot at 5–6 weeks of age ‘sleep through the night’ at 12 weeks of age. This adds to evidence that the two types of problematic behaviour have different causes, and that infant sleep-waking problems usually involve maintenance of signalling behaviours rather than a generalised disturbance.