Ah, daydreaming. Is there anything more pleasant than sitting back and letting your thoughts drift? Well, yes: not letting your thoughts drift, for one. Because according to a study published in the journal Science, people are least happy when their minds wander. Humans, to a degree unmatched by other animals, are capable of thinking about things outside the here and now—something that happened yesterday, or something they hope will happen tomorrow. It's that sort of itinerant intellect that allows us to plan and to learn. But at what cost? Psychologists at Harvard used an iPhone app to find out. At random times throughout the day, the program asked some 2,200 participants what they were doing, what they were thinking about and how they felt. Turns out that people spend nearly half their waking hours thinking about something other than what they're doing. And that whether and where their thoughts tend to stray is a better predictor of their feelings than what they're actually up to. The scientists conclude that a wandering mind is an unhappy mind. So try to focus on, and live in, the present. You might discover that happiness is just being where it's happening.