From the publication:
The study, conducted by Plamen Penev, MD, PhD, of the University of Chicago, focused on 12 healthy men between the ages of 64 and 74. Three morning blood samples were pooled for the measurement of total and free testosterone. In addition to overnight laboratory polysomnography, wrist activity monitoring for six-to-nine days were used to determine the amount of nighttime sleep of the participants in everyday life settings.
The main outcome levels were total sleep time and morning testosterone levels. Analyses revealed that the amount of nighttime sleep measured by polysomnography was an independent predictor of the subjects' morning total and free testosterone levels.
“The results of the study raise the possibility that older men who obtain less actual sleep during the night have lower blood testosterone levels in the morning,” said Penev. “Although the findings suggest that how long a person sleeps may be an indicator of age-related changes in important hormone signals in the body, future studies are needed to determine the importance of these relationships for the health of older adults.”