The research on new mother sleep deprivation contains one finding that tends to surprise formula-feeding mothers: breastfeeding mothers, on average, get more sleep than their formula-feeding counterparts, not less.

The counterintuitive result emerges from several factors. Breastfeeding, particularly during the early weeks, is associated with hormonal changes — notably elevated prolactin levels — that appear to help mothers fall back to sleep more quickly after night feedings. Formula-fed babies, who digest formula more slowly than breast milk, wake less frequently in the first weeks but may wake for longer periods when they do. Breastfeeding mothers, who can nurse without fully waking, often return to sleep within minutes of finishing a feeding.

The larger picture is uniformly difficult. Both groups of new mothers are significantly sleep-deprived by any objective measure during the first months of an infant's life. The cognitive effects are well documented: reduced reaction time, impaired judgment, emotional dysregulation, and memory problems that can persist for years after the period of acute sleep deprivation ends. One study found measurable differences in the gray matter of new mothers' brains that persisted for at least two years postpartum.

What the research makes clear is that postpartum sleep deprivation is not merely an inconvenience but a genuine health issue for new mothers, one that affects their physical recovery from childbirth, their mental health, and their capacity to provide responsive care to their infants. The cultural norm of treating it as a badge of honor — "sleep when the baby sleeps," offered as the entirety of available advice — is inadequate to the scale of the problem.

Partners, extended family, and, in some countries, public health policy, have roles to play in ensuring that new mothers can sleep. The biology is not optional. Neither should the support be.