Neuroscientist David Eagleman explains how your brain perceives time (retrospectively).

David Eagleman: So this is an area of interest to me and my lab’s been studying this for a while, is why time is rubbery and can speed up or slow down.  And it turns out, when I looked into the literature on this, the experiment had never been done about why time seems to move in slow motion when you’re in a life threatening situation.  But I talked to so many people and I’d experienced it myself that I wanted to study that.  So I found a way to study it by dropping people from 150 foot tall tower and measuring their time perception on the way down.  And that, plus several other experiments we did in my lab, led me to understand that people don't actually see time in slow motion during an event.  Instead, it’s a completely retrospective assessment.

In other words, when you’re in a life threatening situation, your brain writes down memory much more densely, and then retrospectively, when you look at that, you have so many details that you don't normally have that it seems as though it must have lasted a very long time.

That's the only interpretation your brain can make.  So time, your assessment of how long something took, has a lot to do with how much energy your brain has to burn during the event and how much footage you have of the event.

Directed / Produced by Jonathan Fowler and Elizabeth Rodd