Data from a small study of healthy young men showed testosterone levels were lower after sleep loss than after more appropriate rest intervals, with the effect of restricted sleep especially apparent between 2 pm and 10 pm.

The majority of the daily testosterone release in men occurs during sleep, the researchers noted, and sleep fragmentation and obstructive sleep apnea are associated with reduced levels of the hormone. In addition to being a critical component of male sexual behavior and reproduction, testosterone also benefits muscle mass and strength, adiposity, bone density, vigor and well-being.

Normal aging is associated with a decrease in testosterone levels of 1% to 2% per year. In this study of 10 healthy men (mean age 24.3 years), daytime testosterone levels fell by 10% to 15% when the subjects had only five hours of sleep per night for one week — a sleep pattern experienced by at least 15% of the U.S. working population. Although the testosterone decline was associated with lower vigor scores, levels of cortisol — which can inhibit gonadal function — did not increase.

The investigators call for further research to determine whether sleep duration should be integrated into the evaluation of androgen deficiency.

JAMA. 2011;305:2173-2174