Prolactin, traditionally regarded as a lactation hormone, is now understood to be a multifunctional modulator of physiology and behavior, integrating reproductive, metabolic, immune, and affective processes. A defining feature of prolactin is its circadian rhythmicity: in humans, levels peak during the early hours of the night-between 2:00 and 4:00 a.m.-a time closely aligned with caregiving, physical intimacy, and co-sleeping. This review synthesizes the neuroendocrine mechanisms underlying prolactin's secretion, highlighting the roles of dopaminergic inhibition, hypothalamic signaling, photoperiodic input, and circadian regulation via the suprachiasmatic nucleus, alongside interactions with sleep-wake cycles. Beyond its well-established physiological roles, prolactin appears to attenuate activity in the hypothalamic-pituitary-adrenal axis, promote neural plasticity, and enhance social bonding in mothers and fathers, as well as in non-parental cooperative caregivers. Integrating these data, we propose that prolactin's nocturnal rise may scaffold a circadian-gated neurobehavioral state - what we term the affiliative mind - characterized by calm affect, empathic attunement, and approach-oriented prosocial motivation. This state is potentiated by oxytocin and endorphin release during close social interactions, forming an amplifying feedback loop. As part of our general hypothesis, we also suggest that prolactin may influence higher-order social cognition through effects on the brain's default mode network, and that its rhythms may become partially synchronized among individuals who cohabitate. Evolutionarily, prolactin's nocturnal action may have transformed nighttime vulnerabilities into opportunities for solidifying trust and group cohesion. Together, plausible extensions of available data position prolactin as a chronobiological scaffold for affiliative behavior within and across individuals and social groups.
Circadian Biology