To test the hypothesis that infant night waking is an adaptation to increase interbirth intervals (i.e., the time between a mother’s consecutive births) by exhausting the mother, we investigated whether maternal sleep disturbance is associated with longer interbirth intervals. We also explored whether postpartum depression symptoms mediated the association between maternal sleep disturbance and interbirth intervals. We used retrospective self-reports from 729 mothers living in Finland. We conducted structural regressions separately for the mother’s two first children at two different age intervals (0–1 and 1–3 years). Infant night waking was associated with maternal sleep disturbance (β = .78–.84) and maternal sleep disturbance was associated with postpartum depression symptoms (β = .69–.81). Postpartum depression symptoms were also associated with longer interbirth intervals for the first child (β = .23–.28). This result supports the notion that postpartum depression in and of itself could be viewed as adaptive from the offspring’s fitness’ point of view, and not just as an unintentional byproduct of the mother’s sleep disturbance. Contrary to our prediction, maternal sleep disturbance was, however, associated with shorter interbirth intervals for the first child (β = -.22– -.30). We discuss the potential role of social support as an explanation for this unexpected result.

To test the hypothesis that infant night waking is an adaptation to increase interbirth intervals (IBIs) (i.e., the time between a mother’s consecutive births) by exhausting the mother, we made an initial attempt at investigating whether maternal sleep disturbance is associated with longer IBIs. We also explored whether postpartum depression symptoms mediated the association between maternal sleep disturbance and IBI length. We used retrospective self-reports from 729 mothers living in Finland. We conducted structural regressions separately for the mother’s two first children at two different age intervals (0–1 and 1–3 years). Infant night waking was associated with maternal sleep disturbance (β  =  .78–.84) and maternal sleep disturbance was associated with postpartum depression symptoms (β  =  .69–.81). Postpartum depression symptoms were also associated with longer IBIs for the first child (β  =  .23–.28). This result supports the notion that postpartum depression in and of itself could be viewed as adaptive for the offspring’s fitness, and not just as an unintentional byproduct of the mother’s sleep disturbance. Contrary to our prediction, maternal sleep disturbance was, however, associated with shorter IBIs for the first child (β  =  −.22 to −.30) when including postpartum depression symptoms in the model. We discuss the potential role of social support as an explanation for this unexpected result.