One of the core ideas I took from Moonwalking with Einstein, one of my favorite books of 2011, was the tight relationship between memory and our perception of how quickly time passes. The more you fall into routine, the more your brain chunks those blocks of time, and thus the faster time seems to fly by. Break up patterns in your life, introduce variety, and time slows.

I haven't seen an actual description of the mechanism by which that works until now, but this is a good one from professor of biochemistry William Reville:

Biological cycles are measured by an internal clock that emits steady signals. The signals emitted over a given interval are counted by something called an “accumulator”. The counts can be stored in memory by an animal and used to repeat certain durations by counting signals until they match the count stored in the memory. No awareness of the passage of time is necessary. Humans however are aware of the passage of time and are easily influenced by attentional demands over a target interval.

Humans have an “attentional gate” through which the signals from the clock must pass in order to reach the accumulator. If the individual decides that the passage of time is important , then the attentional gate is opened wide and signal accumulation is maximised. If the passage of time is unimportant then the gate is narrowed and fewer signals are accumulated. Assuming that the estimate of time duration depends on the count registered by the accumulator, it is easy to see that the same objective time duration, eg 15 minutes, will seem longer when waiting for interview that while relaxing. And, memorising the complex figure requires more attentional resources than memorising the circle, leading to a narrower gate and a lower signal count.

Well worth reading the whole thing, it's not very long, and its power is in how it helps to explain all sorts of time perception phenomena. When Reville follows up his description with this homework assignment, suddenly everything makes sense:

Using the attentional gate model of prospective timing, explain why “a watched pot never boils”, why earthquakes feel longer than they are, and why the “return trip” always feels shorter.