From an article on The Cut:
People who attach dollar signs to their time — or “value time like money” — tend to be overwhelmingly less happy than those who don’t, because their nonworking hours suddenly seem less important. “Free” time gets tainted with guilt because there’s a cost associated with it.
[…]
Other studies found that billing by the hour — no matter how much people charged — compounded the tendency to view time and money as one and the same. Those who did so were less likely to take pleasure in leisure activities, because they were too preoccupied by the opportunity cost of their time.
I know I’ve experienced these issues as a freelancer.
Have you?
Written by 
