Prolactin is a polypeptide hormone produced primarily by specialized cells called lactotrophs within the anterior pituitary gland. While present in both males and females, its main role is stimulating the development of mammary glands and initiating milk production following childbirth (lactation). Prolactin also helps regulate numerous bodily functions, including metabolism, immune response, and behavior.
Prolactin’s Link to Drowsiness
Prolactin generally causes sleepiness, especially when the hormone is released in a rapid surge. This effect is most commonly observed following breastfeeding, where the act of suckling triggers a sharp release of prolactin that results in relaxation and drowsiness in the mother. This immediate post-nursing sedation is thought to be an evolutionary adaptation that promotes rest and energy conservation for the new parent. Prolactin’s association with rest is also evident in the natural human sleep cycle. Levels naturally increase during sleep, reaching their highest concentration in the early morning hours, suggesting involvement in regulating the sleep-wake cycle.
The Neurochemical Mechanism of Sedation
The sedative effect of prolactin is tied to its interaction with key neurotransmitters in the central nervous system. Dopamine, a neurotransmitter associated with wakefulness, is the main chemical inhibitor of prolactin secretion. The hypothalamus releases dopamine, which constantly suppresses prolactin release from the pituitary gland. When prolactin levels surge, the hormone exerts a dampening effect on the dopamine system in the brain. By reducing the influence of the stimulating dopamine pathway, prolactin promotes reduced alertness and lethargy.
Prolactin also interacts with gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain. GABA slows down brain activity, producing a calming effect. Prolactin’s influence on GABA-related pathways contributes to the overall sedative state when the hormone’s concentration is high.
When Prolactin Levels Are Elevated
Prolactin levels can be elevated through several physiological and pathological circumstances beyond nursing. Physiologically, the hormone is naturally high during pregnancy and lactation, as well as during periods of physical or psychological stress. Intense exercise and sexual activity can also cause temporary increases in circulating prolactin.
Sustained, non-physiological elevation, known as hyperprolactinemia, can occur due to medical causes. A common pathological cause is a prolactinoma, a non-cancerous tumor of the pituitary gland that produces excessive amounts of the hormone.
Certain medications can also lead to hyperprolactinemia by interfering with dopamine signaling. Antipsychotic drugs and some antidepressants block dopamine receptors, removing the natural brake on prolactin secretion. High prolactin levels can also be a side effect of chronic kidney disease and hypothyroidism, potentially contributing to unexplained fatigue and sleep disruptions.