Keith Negley for NPR

The human Y chromosome (left) holds the code for "maleness"; that's the X on the right. Andrew Syred/Science Source hide caption

itoggle caption Andrew Syred/Science Source

Testosterone levels in men can go up and down throughout the day. Katherine Streeter for NPR hide caption

itoggle caption Katherine Streeter for NPR

Katherine Streeter for NPR Katherine Streeter for NPR hide caption

itoggle caption Katherine Streeter for NPR

Some men take testosterone hoping to boost energy and libido, or to build strength. But at what risk? iStockphoto hide caption

itoggle caption iStockphoto

There's more than one reason to aim for a good night's sleep. Charles Taylor/iStockphoto.com hide caption

itoggle caption Charles Taylor/iStockphoto.com

The active ingredient in Propecia, a baldness remedy approved by the Food and Drug Administration in 1997, is showing new promise as a way to prevent some prostate cancers. AP hide caption

itoggle caption AP

Skipping breakfast is risky. iphoto hide caption

itoggle caption iphoto

A billboard advertising robotic surgery hangs outside Boston's Fenway Park in 2009. Hospitals and doctors have heavily promoted robotic surgery. Charles Krupa/AP hide caption

itoggle caption Charles Krupa/AP

Prescriptions for testosterone medications, such as Testim and AndroGel, to men 40 and older rose threefold over the decade ending in 2011 MICHAEL BRYANT/MCT/Landov hide caption

itoggle caption MICHAEL BRYANT/MCT/Landov

A man smokes a cigarette as he takes a break at a fruit market in Hyderabad, India. Smoking tobacco is eight times more prevalent among Indian men than women. Noah Seelam/AFP/Getty Images hide caption

itoggle caption Noah Seelam/AFP/Getty Images

If you're interested in buying Viagra, Pfizer, the drug's maker, will now sell it to you from its official website. William Vazquez/AP hide caption

itoggle caption William Vazquez/AP