Increased alertness leads to a higher frame rate of information processing, making time seem slower.

“Typically the more alert you are, the higher the frame rate, your thin slicing your experience. You've probably had this happen. If you're ever very stressed and you're waiting for something or somebody, it seems like it takes forever because your frame rate is higher you're analyzing time more finely. Conversely, if you are very relaxed or even sleepy, you wake up and you have to think of all the things you have to do. It will seem like the world is going by very, very fast and that you are moving very slow.”