Most People are Happier Working than in Their Free Time
scotthyoung.com
scotthyoung.com
I'd argue that it's due to the structure of work life versus home life. At (office) work, you typically have set hours and a certain set of activities to perform - the reality vs expectations gap is quite small.
In your "free" time, however, you expect to have a good time, yet usually have to wash clothes and dishes, cook food, go grocery shopping, and 101 other dull activities that merely wrap around the occasional fun things like watching a movie, going out, having sex, whatever. The reality vs expectation gap in your free time is gigantic for many people.
For most people, being engaged in work gets them to that state of living in the present, where the mind is fully focused on a task and doesn't get a chance to explore itself.
For me though, being truly happy is being able to relax in that free time, soak up the present "nothingness", allow the mind to wander, take everything as it is (without judgment or assumption), and just be grateful for everything I've been blessed with (even if there are negative circumstances).
It would be interesting to repeat the same study for people who are financially secure.
W.B. Yeats, "Among School Children"