SUMMARY Major physiological changes occur following parturition and the onset of lactation, including the withdrawal of oestrogen and progesterone, with a consequent increase in circulating prolactin (PRL). Changes in other circulating hormones are well known to alter sleep architecture in other circumstances. We therefore aimed to assess whether sleep architecture is altered in fully lactating women as a result of hormonal changes associated with lactation. A descriptive comparison study was undertaken on 12 fully breastfeeding women (B/F), 12 age‐matched control women (CTRL), and seven postnatal women who had chosen to bottle‐feed their infants (BOTTLE). Maternal age, infant age and body mass index (BMI) were similar between all three groups. We performed overnight polysomnography utilizing the Portable Compumedics P‐series. The total sleep time (TST) and rapid eye movement (REM) sleep time were similar in the three groups of women. However, B/F women demonstrated a marked increase in slow wave sleep (SWS), 182 ± 41 min compared with CTRL (86 ± 22 min, P < 0.001 compared with B/F) and BOTTLE subjects (63 ± 29 min, P < 0.001 compared with B/F). There was a compensatory reduction in light non‐rapid eye movement (NREM) sleep in B/F when compared with CTRL and BOTTLE. The most likely explanation for the altered sleep architecture noted to occur in women who are fully breastfeeding their infants is an increase in circulating PRL, which occurs in lactating women. Enhanced SWS may be another important factor to support breastfeeding in the postnatal period.