- Objective Controversies exist regarding the impact of co-sleeping on infant sleep quality. In this context, the current study examined: (a) the differences in objective and subjective sleep patterns between co-sleeping (mostly room-sharing) and solitary sleeping mother–infant dyads; (b) the predictive links between maternal sleep during pregnancy and postnatal sleeping arrangement; (c) the bi-directional prospective associations between sleeping arrangement and infant/maternal sleep quality at three and six months postpartum. Methods The sample included 153 families recruited during pregnancy. Data were obtained in home settings during the third trimester of pregnancy and at three and six months postpartum. Mothers were asked to monitor their own sleep and their infants' sleep for five nights using actigraphy and sleep diaries. Questionnaires were used to assess sleeping arrangements, feeding methods, socio-demographic characteristics, and maternal depressive and anxiety symptoms. Results Mothers of co-sleeping infants reported more infant night-wakings than mothers of solitary sleeping infants. However, none of the objective sleep measures was significantly different between co-sleeping and solitary sleeping infants, after controlling for feeding techniques. Co-sleeping mothers had significantly more objective and subjective sleep disturbances than mothers in the solitary sleeping group. Moreover, poorer maternal sleep during pregnancy and at three months postpartum predicted higher levels of co-sleeping at six months. Conclusion Mothers of co-sleeping infants report more infant night-wakings, and experience poorer sleep than mothers of solitary sleeping infants. The quality of maternal sleep should be taken into clinical consideration when parents consult about co-sleeping.