Here is a hypothesis that changes how you might think about the hardest hours of new parenthood.
A 2025 comprehensive review by Mendoza et al. proposes that prolactin's dramatic nocturnal surge — reliably peaking between 2 and 4 AM in breastfeeding women — may not be primarily about milk production at all. The authors argue it could be an evolutionary mechanism that transformed darkness, isolation, and the demands of a newborn into something neurobiologically tolerable. Their term for it: a "calm affiliative neurobehavioral state." The idea is that the body floods itself with prolactin at night not just to keep the milk supply going, but to make caregiving at 2 AM feel fundamentally different from caregiving at 2 PM. Warmer. More sustainable. Less threatening to survival.
Whether or not this hypothesis holds up to further scrutiny — and it is a hypothesis — it reframes what you are experiencing in those night feeds as something your biology is actively shaping, not just something you are enduring.
The Clock Running the Surge Isn't Your Baby
This is the part that surprises most people: the nocturnal prolactin peak is not driven primarily by your infant's feeding schedule. It is driven by your brain.
The suprachiasmatic nucleus (SCN) — the brain's master circadian clock — governs the surge through two mechanisms Mendoza et al. detail: dopaminergic inhibition and melatonin signaling. During the day, dopamine suppresses prolactin release from the pituitary. At night, that suppression eases, and melatonin signaling reinforces the timing. The result is a predictable, clock-driven elevation that happens whether or not a feeding occurs around that time.
The molecular evidence for this is striking. Poletini et al. (2010) knocked down clock genes in SCN neurons of rats and abolished both the diurnal and nocturnal prolactin surges entirely — without altering any suckling stimuli. (This is animal research, but the architecture it describes is consistent with what's observed in lactating humans.) The prolactin rhythm wasn't downstream of feeding. It was upstream of it, baked into the circadian architecture of the brain.
This matters because it means the nocturnal peak is robust. Díaz et al. (1989) tracked basal plasma prolactin in exclusively breastfeeding women across the first postpartum year and found a consistent circadian peak between midnight and 6 AM — month after month. The same study found that women with lower nocturnal prolactin were more likely to resume menstrual cycles before six months, even when their nursing frequency was identical. The nocturnal peak isn't a curiosity. It appears to be doing real physiological work.
What suckling adds is a different kind of signal. Glasier et al. (1996) documented that every nursing episode triggers its own prolactin pulse, regardless of time of day — but when feedings are infrequent, prolactin drops back to baseline between them. The circadian surge and the suckling-driven pulse are two separate systems. Night feeds ride the surge. Daytime feeds generate their own smaller spikes. Both matter. Neither replaces the other.
The WHO/NCBI clinical lactation review notes something important in the practical context: the tight relationship between prolactin levels and milk volume is most critical in the early weeks of lactation. After supply is established, the breast becomes more autonomous — local autocrine signals regulate how much milk is made based on how completely the breast is drained. Prolactin remains relevant, but its role shifts.
So Do You Have to Feed at 2 AM?
The honest answer depends on where you are in the postpartum timeline — and what question you're actually asking.
In the first six to eight weeks, when milk supply is still being calibrated, the nocturnal peak has genuine practical significance. A feeding at the peak amplifies the prolactin pulse on top of an already elevated baseline. The WHO/NCBI review is explicit: nighttime breastfeeding is especially helpful for maintaining milk supply. Skipping nights consistently during this window can reduce overall prolactin exposure at exactly the window where it's most potent.
After supply is established, the calculus shifts. The circadian surge still happens. Your brain is still running the clock. But your body's milk output becomes less dependent on each individual prolactin reading and more dependent on consistent, adequate drainage across the day. Missing an occasional night feed will not collapse your supply if the breast is being well-drained at other times.
There is also a counterintuitive finding worth knowing. Doan et al. (2014) found that exclusively breastfeeding mothers obtained about 30 more minutes of total nocturnal sleep per night compared to formula-feeding mothers — likely because they could respond to feeds without fully waking to prepare a bottle and then fell back asleep faster. The same study raised the hypothesis that sleep deprivation itself may interfere with prolactin release — a finding the authors treated as preliminary rather than settled. Fragmented, insufficient sleep doesn't just feel bad; it may undermine the very hormonal environment you're trying to protect.
The practical framework we use at Imprint for evaluating these tradeoffs: weigh the input (a night feed) against both the direct biological benefit and the cost to the system providing it. Prolactin peaks at night. It also requires a functional, not-fully-depleted mother to do what evolution designed it to do.
You don't necessarily have to feed at 2 AM forever. But your body is doing something real at that hour — something that appears to have been selected for over a very long time. The question isn't whether to ignore it. It's how to work with it without breaking yourself in the process.
That distinction is probably what evolution was building toward anyway.
