Just 2 nights of restless sleep can make you feel a shocking number of years older, experts say

So this is why they call it beauty sleep.

Getting inadequate shut-eye can make you feel older than you actually are, according to new research.

The two-part study, published Wednesday in the Proceedings of Royal Society B, found that poor slumber can tack on more than four years to how old people feel, while getting ample rest has the opposite effect, making people feel more rejuvenated and youthful.

Lack of sleep has been associated with adverse health outcomes, such as poor mental health, Type 2 diabetes and heart disease.

“Age is more than just the perception,” study author Leonie Balter, of the Karolinska Institute in Sweden, told New Scientist. “We know those who feel younger than their actual age live healthier and longer.”

In the first part of the study, the researchers surveyed 429 people aged 18 to 70, asking how old they feel and how many days out of the past 30 that they felt they slept poorly.