It's no secret that Elon Musk puts a tremendous number of hours into his work, famously working upwards of 120 hours per week, and sharing his opinion that founders need to work 80-plus hours a week. The astronomical number leaves many wondering how he finds time to sleep and exhausted by the thought of working so much. It appears as though the founder of multiple companies and father of seven has more hours in the day than others. And, of course, while this isn't so, there is something Musk has that others don't. 

Those who put in a great deal of time per week, week after week, are those who have one free thing that others don't. It's not that they have the superhuman ability to survive without sleep, have incredible time management skills, or are powered by a tremendous amount of motivation. 

They have fun. 

The only way a founder can sustain working an incredibly high number of hours every week, month after month, is by having fun. And psychology proves it. 

We all have the same number of hours in a day, and seconds in minutes, but that doesn't mean we all experience hours, minutes, or even seconds the same way. Psychologically, time passes very differently depending on our emotional state. In fact, when under stress, our perception of time actually slows down. 

It's why, when giving a short speech or presentation, seconds feel like minutes, meanwhile, hours pass like minutes during dinner among great company. It's also why the world's top boxing champions are known for unbelievably fast reactions. So fast that to onlookers, it appears as though boxers preemptively know their opponent's next move. In reality, they don't know what they're going to do next; they see what they're already doing. But because they're under such stress, their perception of time is in slow motion, enabling them to see what others cannot.  

Founders who are having fun experience time differently, so they're able to work more without the physical or mental effects typically associated with long working hours. When time is spent having fun, the mental state doesn't deteriorate like it would under stressful or dissatisfying conditions. 

John Mackey, who built the $14 billion supermarket empire Whole Foods from the ground up, said that it "wasn't work, it was play." The founder, who worked 80 hours a week in the early days, still works a great deal--even after becoming a millionaire and as he pushes 70 years old. Because neither money nor age will stop you from doing something if it is fun. 

Instead, it actually serves to energize. So while the idea of working 80 or even 120 hours per week sounds draining to many, for those having fun, it is exhilarating. 

Founders are expected to have a ton of passion and drive, but we don't necessarily expect founders to be having a ton of fun in the process of building a successful startup, which involves a tremendous amount of hours, little sleep, and missed soccer games. But founders need to have fun to fuel their ability to invest a great deal of energy into their startup. 

In nearly any other field, a lot of work isn't exactly synonymous with a lot of fun. But in entrepreneurship, hard work is fun work. And to be successful and launch like Elon Musk, you need to have fun doing hard things. Because those who have fun doing hard things are those who do the hardest thing: build a successful startup.