STOCKHOLM, March 28 -- It's often said that age is all in the mind. But a study in Sweden shows that feeling young is not just a matter of subjective perception. It's also a question of sleep.

You may have looked at yourself in the mirror and felt as if you didn't "look your age." This is because we don't always have a clear idea of our biological age. American researchers have previously discovered that, from the age of 30 onwards, most people tend to underestimate their age. In other words, they think they're younger than they really are.

While this discrepancy between subjective age and biological age may sound trivial, it can actually have serious medical consequences. Indeed, the illusion of youth is associated with better cognitiv...