Losing sleep might sap men’s testosterone, according to new research by Chicago scientists. Men release most of their testosterone during sleep, so University of Chicago professor Eve Van Cauter wanted to see if less sleep meant less of the hormone.

She recruited volunteers and restricted their sleep to just five hours per night for eight days. At the end of that stretch, her team took blood samples to test for hormone levels. They found an average 15 percent drop in testosterone.

“And 15 percent is not an insignificant amount, since it is about the amount that occurs with normal aging by 10 to 15 years,” Van Cauten said.

So in one respect, a week of bad sleep is like aging a decade or more.

Testosterone plays a role in reproductive health, but also in insulin sensitivity, bone density and general vigor. Scientists are interested in how the reduction might affect everything from fertility to weight gain and diabetes.

The study sample was small – just 10 subjects – and limited to lean, healthy young men. There are many open questions, like how the effect might change over time and how it translates to women and older or overweight subjects. But the finding, out in the Journal of the American Medical Association, adds to growing evidence that sleep deprivation has big effects on hormone levels.