You cannot ask somebody to be creative in 15 minutes and really think about a problem. You might have a quick idea, but to be in deep thought about a problem and really consider a problem carefully, you need long stretches of uninterrupted time.

I think that sleep and work are very closely related - not because you can work while you're sleeping and sleep while you're working. That's not really what I mean. I'm talking specifically about the fact that sleep and work are phase-based, or stage-based, events.

Very, very few people actually have long stretches of uninterrupted time at an office.

Jason Fried Quotes