Why Does Time Fly?
http://www.scientificamerican.com/article.cfm?id=why-does-time-fly The more energy it takes to process a stimulus the longer it appears as a subjective experience of time. Something moving toward you has more relevance than the same stimulus moving away from you: You may need to prepare somehow; time seems to move more slowly. When we experience something as “taking a long time” it is really the result of three inter-twined processes: the actual duration of the event, how we feel about the event, and whether we think the event is approaching us. There is little we can do about the first factor but there are obvious ways of modulating how we feel about an event and how we think about an event approaching us.