It has been proposed that infant night waking is an adaptation by which the infant exhausts the mother and increases the length of her interbirth intervals to decrease sibling rivalry over maternal investment. Based on this premise, the current study examined (i) whether maternal sleep disruption due to infant night waking is associated with increased interbirth intervals and symptoms of postpartum depression, and (ii) whether symptoms of postpartum depression mediate this association. We analyzed the associations between infant night waking, maternal sleep disruption, symptoms of postpartum depression, and interbirth intervals, using retrospective data from 729 mothers living in Finland. We conducted five structural regressions, one with the full sample, two for the oldest child at the age intervals 0 – 1 years and 1 – 3 years, and two for the second oldest child at the same age intervals. For the full sample, infant night waking was associated with maternal sleep disruption (β = .84, 95% CI [.82; .86]), and maternal sleep disruption was associated with symptoms of postpartum depression (β = .75, 95% CI [.72;.77]). Symptoms of postpartum depression were associated with longer interbirth intervals (β =.23, 95% CI [.12; .35]), and maternal sleep disruption was associated with shorter interbirth intervals (β = -.30, 95% CI [-.41; -.19.]). The results indicate that maternal sleep disruption due to infant night waking is not associated with longer interbirth intervals. Symptoms of postpartum depression are, however, associated with longer interbirth intervals. This indicates that they might play a more active role, from the offspring’s perspective, in lengthening the interbirth intervals, and not just being an unintentional byproduct of the mother’s sleep disruption.