Annalee Newitz in io9:
When you watch the seconds tick by on a digital watch, you are in the realm of objective time, where a minute-long interval is always 60 seconds. But to your brain, a minute is relative. Sometimes it takes forever for a minute to be over. That's because you measure time with a highly subjective biological clock.
Your internal clock is just like that digital watch in some ways. It measures time in what scientists call pulses. Those pulses are accumulated, then stored in your memory as a time interval. Now, here's where things get weird. Your biological clock can be sped up or slowed down anything from drugs to the way you pay attention. If it takes you 60 seconds to cross the street, your internal clock might register that as 50 pulses if you're feeling sleepy. But it might last 100 pulses if you've just drunk an espresso. That's because stimulants literally speed up the clock in your brain (more on that later). When your brain stores those two memories of the objective minute it took to cross the street, it winds up with memories of two different time intervals.
And yet, we all have an intuitive sense of how long it takes to cross a street. But how do we know, if every time we do something it feels like it a slightly different amount of time? The answer, says neuroscientist Warren Meck, is “a Gaussian distribution” – in other words, the points on a bell curve. Every time you want to figure out how long something is going to take, your brain samples from those time interval memories and picks one. “You randomly sample from it,” says Meck. “So you might pull a 25 out of distribution, or a 36. You're only accurate in the mean.”
The good news is that, on average, you will predict correctly how long it takes to cross the street. The bad news is that occasionally, you'll pull an outlier memory from that bell curve and decide to cross the street much more slowly than you should.