Testosterone production naturally declines as men age, but a new study seems to show that another symptom of aging might be the actual culprit. Researchers at the National University of Singapore found that lack of sleep—a natural consequence of aging for many men—drastically reduces the amount of testosterone that is produced. Scientists studied 531 exceedingly healthy men (mean-ing they had no other reasons for lagging testosterone levels) and discovered that one extra hour of sleep a night raised testosterone levels by 12 percent. If a supplement guaranteed a 12 percent in increase in testosterone output, it would fly off the shelves. This is just one more piece of evidence why sleep is such an important piece in the burn fat/build muscle puzzle. Adequate sleep has already been shown to lower body fat levels, boost mood, and improve memory.