I'm sorry, but this is clickbait junk science. > Psychologists in Sweden found that, on average, volunteers felt more than four years older when they were rest

I'm sorry, but this is clickbait junk science. > Psychologists in Sweden found that, on average, volunteers felt more than four years older when they were restricted to only four hours of sleep for two consecutive nights, with some claiming the sleepiness made them feel decades older. The average was four years older, with some people claiming they felt "decades" older. This should be enough to tell you how useless this methodology is. People cannot reliably estimate how much "older" they feel because it's their subjective interpretation of an experience they don't have. By definition they don't know what being 4 years older feels like. You might as well ask them whether sleep deprivation made them feel like they'd been waterboarded for 4 hours or 4 days.