Flow (Mihaly Csikszentmihalyi) is a state where one is entirely consumed in an activity. In such a state one is generally not aware of his very existence. Yet Csikszentmihalyi holds it is a central way to a happy life.
Why is not everybody trying to acheive as much flow as possible? (aside from the regular reasons why people do not improve their lives.....)
Possibly because in flow we are nott conscious of our very existence and therefore we are later less aware of the fact that we had so much fun (or positive affect ;-))
Being happy and not knowing about it is highly interesting idea. I had two periods in my life when I was very anxious etc. but other people thoguht that I looked as happy as never before and after. (I am still not sure how to accout ofr htese experiences....)
A similar idea is promoted by Daniel Kahneman with his observation that moment to moment experience differs from the way we think about our life.
One may similarly wonder what is the meaning and value of happiness at sleep.