system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening when time appears to be passing slowly, or faster than usual?

Why do we sometimes sit in class waiting ages for 5 minutes to pass, but when I sit down to study for a test and its already 2am?
gpt
How you perceive time deals with how much information you are taking in or how involved you are in a situation. Time flies when your having fun, or in your case, studying. Your brain is busy with things that don't involve keeping track of time, so it doesn't. 

There is prospective time and retrospective time. Prospective time is how you perceive time when you are studying or looking at the clock waiting for time to pass. Retrospective time is looking back at how long it actually felt. While doing something interesting or involving, your brain is taking in heaps of information and not keeping track of time. But when you remember events or occurrences, the more information you have, the longer it seems. For example, prospectively, while going on an exciting trip, time seems to fly. Retrospectively, an exciting trip of two days can feel like a week.