If you're not as active in bed as you used to be, maybe it's because you're not as inactive in bed as you need to be.

Confused? Don't be: A new study finds that men who get too little sleep have low levels of testosterone.

And if you've been running on short rest yourself, take heed -- because those sex-robbing effects can kick in after a single week of missed sleep.

Researchers asked 10 healthy men to get eight hours of sleep a night for a week at home before spending 11 nights in a sleep lab, where they snoozed for 10 hours a night for the first three nights... and then five hours a night for eight nights.

Don't worry, this isn't about to turn into a math quiz. The only number you need to remember here is 15 -- because the researchers found that testosterone levels plunged by up to 15 percent following the week of five-hour nights.

The men also saw drops in "vigor" after missing out on sleep -- although I suppose that could have just as easily come from being tired and worn out as much as the reduced testosterone levels.

The takeaway from the study in the Journal of the American Medical Association, of course, is that you should get your rest -- but that's not the only way to keep your testosterone tank topped off.

Another new study finds that obese men who suffer from low testosterone can get their mojo back after weight-loss surgery.

Researchers tracked 75 men who were seen at an obesity clinic between 2007 and 2010 and confirmed what we could have guessed: The heaviest men had the lowest testosterone levels.

Then, 17 underwent gastric bypass surgery – and researchers say that a year later, these men had significantly higher levels of testosterone and were all within the "normal" range.

That may sound good, but don't rush to go under the knife just yet -- because the real key here isn't the surgery.

It's the weight loss.

And there are better, safer and cheaper ways to reach that goal -- like the time-tested good habits you'll need to learn anyway if you hope to stay healthy enough for regular sex.

But let's say you're slim, trim, and well-rested... but still having production problems in your testosterone factory.

Don't panic -- because you've got options, too.

As I told you recently, an extract made from the Middle Eastern spice fenugreek has been shown to boost testosterone levels, improve libido, and even increase the quality of sex.

You can read more about that right here.