Your internal clock is just like a 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. 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.