Scientists have known for a while now that cutting your calories by at least 15 percent on a daily basis can significantly increase your lifespan, but they haven't been sure why — that is, until now. It turns out, it might have more to do with how your sleep gets affected by the diet, reports New Scientist.

Leanne Redman of Pennington Biomedical Research Center in Louisiana and colleagues recently performed one of the most extensive studies of calorie restriction on humans, which monitored 53 adults over the course of two years. Of those subjects, 34 stuck to a strict diet that limited calories by 15 percent compared to the daily recommended amount. Sure enough, as with previous studies on calorie restrictive diets, these subjects showed positive health signs, such as better blood cholesterol and glucose levels.

More tellingly, though, was another correlation uncovered by the study. Subjects demonstrated a significant drop in their night-time metabolic rates and body temperatures. All in all, metabolic rates during sleep were reduced by 10 percent.

Looking even deeper into the numbers, Redman found that these lowered metabolic rates during sleep resulted in a 20 percent drop in cellular oxidative stress, and that's the key. Metabolism is essentially the process that keeps us alive — it's what converts what we eat into energy. But the faster and harder our metabolisms work, the more wear and tear we accumulate in our cells. This is what cellular oxidative stress measures, and long term, this is part of what causes us to age.

So if consuming fewer calories causes our bodies to conserve energy rather than burn it off quickly, then that ought to lead to slower aging. At least, that's the theory at play here.

One of the best times to conserve energy is during sleep, so it makes sense that our bodies should choose this time to adjust our metabolic rates to account for the lower levels of calories being consumed. This is why some animals hibernate, for instance.

“This study is the first to show that humans respond to calorie restriction by a reduction in resting metabolic rate,” explained Luigi Fontana of Washington University in Missouri.

Of course, lower metabolic rates can also result in bouts of sluggishness, loss of libido, and regularly feeling cold, so it's a tradeoff. Calorie restrictive diets certainly aren't for everyone, and it's also important to make sure that any restricted diet does not also mean limited nutrients. A balanced diet is important no matter how many calories you're consuming.

New study finally explains why calorie restriction can extend your lifespan
The benefit of cutting calories may have more to do with how your sleep gets affected, new research finds.