Work is the same way for two reasons. The enjoyment of work—to the extent that you have any—is likely highest in the first hours of the day when you are fresh, not tired, working on the most important things. By the eighth, 10th, 12th hour of the day, it’s a lot less fun. Of course, work also provides you with income. But the value of this also declines as you add more work. Think about it like this: The first hour of work buys you food, the second buys you housing, and so on, but the 12th hour might be buying you a nicer espresso maker. Everyone likes nice espresso, but the value of the income decreases as you get more of it. (This is called “decreasing marginal utility of consumption.”)
How quickly your enjoyment of any activity declines is pretty personal. You may still love that third hour of stickers, or you may really hate the second hour of work. But, in general, for nearly everyone, there seems to be at least some decrease in enjoyment as you continue an activity. In short, humans are programmed to get bored.
Knowing this, how do you divide your time to make yourself as happy as possible? It’s simple: The last hour of your time doing each activity should contain equal amounts of happiness. If I spend eight hours at work and three with my daughter, then this is ideal if the eighth hour at work has the same amount of happiness as the third hour with her.
How do I know this is right? If it were not the case—if the third hour with my daughter was much better—then I could make myself happier by taking the last hour at work and spending it with my daughter instead.
The key here is understanding that I may value my daughter much, much more than my job and still want to spend more time at work. Because it may be (as it is for me) that the first hour of time with her gives me incredible joy—far outstripping even a whole week of work happiness—but the enjoyment diminishes fairly quickly. If it decreases much faster than the enjoyment of work, then it’s easy to see why I might want to spend more hours at work.
How can you figure out if you would be happier working less? (Short of quitting your job, of course, which may be a costly way to try this out.) Here’s a simple thought experiment: Imagine that the week contains one more waking hour. It magically appears, and you can do what you want with it. What do you want to do? Thinking about it this way forces you to think on the margin: How much is the next minute or hour of time worth? And that marginal thinking is the right way to make decisions.
If the answer is that you’d much rather spend that hour with your family, you are spending too much time at work. If the answer is that you’d much rather spend that hour at work, you are spending too much time with your kids. If the answer is that some weeks you’d work, some you’d stay with the family, then congratulations: You are as happy as you can be.