I am working 40h/week as a software developer for a non-profit... Whether I spend 6h/day thinking hard about problems[1] at my desk or on the sofa or while walking a dog makes no difference w.r.t. but a lot for health and general well-being.

[1] I vehemently believe in the "80% thinking, 10% cooding, 10% testing" idea of software development.