Part of “CHAPTER 6 – ENDOCRINE RHYTHMS“

Under normal conditions, the 24-hour profile of plasma PRL levels (see Chap. 13) follows a bimodal pattern, with minimal concentrations around noon, an afternoon phase of augmented secretion, and a major nocturnal elevation starting shortly after sleep onset and culminating around midsleep. Episodic pulses occur throughout the 24-hour span. Morning awakening is consistently associated with a brief PRL pulse.53,54 Studies on PRL during daytime naps or after shifts of the sleep period have demonstrated that sleep onset is invariably associated with an increase in PRL secretion. This is well illustrated by the profiles shown in Figure 6-2 in the presence and in the absence of sleep and in Figure 6-4, which compares the profiles of day and night workers. However, a sleep-independent circadian component of PRL secretion may be observed in some individuals, particularly in women.54 An example may be seen in the PRL profiles of night workers (see Fig. 6-4) in whom a nocturnal elevation occurred despite nocturnal activity.15