In 2012, University of Chicago endocrinologist Eve Van Cauter subjected a group of healthy, lean men to sleep deprivation in a lab – the guys logged five hours a night – and monitored their testosterone levels. She expected some drop in the sex hormone – earlier work had established that men with sleep apnea are more likely to have "low T" – but was shocked to discover that after a week, these fit dudes had an average 15 percent drop in testosterone. Consider that science's standard T decline is a 1 percent drop per year for men in their late thirties and, Van Cauter says, "we're able to say that sleeping five hours a night, which many people regularly do, corresponds to 15 years of aging in one week." Hormonally speaking, of course. She adds that while a lot of middle-aged men talk to their doctors about testosterone gels to enhance libido and boost energy, "it's possible that an extra hour in bed might do the trick."
Let's be clear. The testosterone number on your lab report isn't the be-all for libido. But to the best of our knowledge, testosterone is the primary biochemical driver of desire. Keep those levels as close to your youthful high-water mark as possible, and you defend your drive against the predations of time, especially if you're one of the 25 percent of men with low T readings, in the 200 to 400 range.
Van Cauter explains that one reason testosterone is so tethered to the amount of sleep you get is because that's when the body produces some 80 percent of its androgens (male hormones and human growth hormones). And most of that production occurs during the first and deepest phase, "slow-wave" sleep. Sadly, this rejuvenating interlude does shrink with age. A guy in his mid-twenties may get 80 to 100 minutes of slow-wave sleep, Van Cauter says. By age 40 that number can be cut in half or more.
The remedy for that is straightforward: exercise. The man who regularly sweats can stave off half of his slow-wave sleep decline, Van Cauter estimates, keeping T levels high. This past spring, Swiss researchers refined the exercise prescription. They found that regular, vigorous exercise increased hormone-enhancing slow-wave sleep by up to 15 percent more than moderate-intensity routines. According to study coauthor Serge Brand, evening workouts, within a couple hours of bed, delivered the biggest bang for the buck.