Chapter 42, Problem 4ITD

Researchers measured prolactin and growth hormone in the blood plasma of six subjects at regular intervals over a 24-hour period. The investigators were interested in determining if the levels of these two hormones cycle over a 24-hour period. The Figure presents the results of their experiments, with the y axis values showing the amount of each hormone expressed as a percentage of the 24-hour mean value, and each point showing the average of the six subjects’ levels.

FIGURE Mean concentrations of prolactin and growth hormone in plasma, averaged for six subjects and expressed as a percentage of the average concentrations measured for the 24-hour period.

Do the results support the hypothesis that the changes in prolactin level over a 24-hour period are associated with the sleep–wake cycle?

Source: From J. F. Sassin et al. 1972. Human prolactin: 24-hour pattern with increased release during sleep. Science 177:1205–1207.