"Fun" is usually defined as something providing instant gratification (through hormonal response), though there is fun in retrospect too ("I was scared like shit, that was fun"). And while it's nothing to sneeze at and we should always have some, you can achieve the similar with different medications or narcotics (if the goal is "have more fun").
But I wouldn't optimize for that: we can achieve plenty in our lives, including having plenty of fun, by just being ourselves.
As in, get the sleep you need. Do the work you must and the work you enjoy. Have the fun you want.
While our lifetimes are short, they are not that short. Even if we got 10% more of the waking hours, that won't be the thing that makes your life worthwhile or not. If you spent the other 90% making it worthwhile, that'll do.
But if you must, go for it!
