Have you ever 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 how old we are biologically. Indeed, US researchers have previously discovered that most people 30 and above tend to think they’re younger than they really are.

This discrepancy between subjective and biological age can have medical consequences. The illusion of youth is associated with better cognitive performance, greater life satisfaction and, more generally, better physical and mental health.

So, what can we do to help us feel young? According to researchers at Stockholm University, the secret could lie in the quality of your sleep.

“Given that sleep is essential for brain function and overall wellbeing, we decided to test whether sleep holds any secrets to preserving a youthful sense of age,” explained Leonie Balter, researcher at the Department of Psychology.

To do this, Balter and colleagues first questioned 429 individuals aged between 18 and 70 about how they felt about their age. They also asked them to estimate the number of nights they had not slept enough in the month preceding the study, as well as their level of sleepiness.

It turned out that, for every night of insufficient sleep, the volunteers felt on average 0.23 years older.

In a second phase, the academics conducted an experiment with 186 participants aged between 18 and 46. They asked them to sleep for just four hours for two nights, and then to have two full nights of sleep, with nine hours of sleep each time.

It turned out that the volunteers felt on average 4.4 times older when they didn’t get enough sleep.

The researchers hypothesise that the effects of sleep on perceived age are linked to the level of sleepiness people felt. Feeling alert made people feel four years younger than their biological age, subjects who struggled to stay awake felt six years older.

These findings, published in the journal Proceedings of the Royal Society B, show the extent to which the quality of our sleep can benefit or undermine mental health.

“Safeguarding our sleep is crucial for maintaining a youthful feeling. This, in turn, may promote a more active lifestyle and encourage behaviours that promote health, as both feeling young and alert are important for our motivation to be active,” Balter concluded.