You already know this feeling. The best vacations often seem over almost as soon as they begin. A forty-minute meeting can feel like half an afternoon. 

Time did not actually change. Your brain did.

Every animal, a new study has found, lives inside its own version of time, shaped by biology and built for survival. Scientists have called it a "timescape." Humans have one too, and it is more flexible than you might expect.

It shifts with your emotions. Dopamine, the brain's reward chemical, speeds up the internal clock so that time feels like it is passing faster. This is why a three-hour dinner with good friends is over before you have looked at your watch. 

Norepinephrine, released under stress or danger, slows the clock down, which is why a frightening moment can stretch into something that feels almost suspended. During a car accident, people commonly report that time seemed to slow to a crawl, possibly an evolutionary advantage that allowed quick decisions at exactly the right moment.

It shifts with your age. As we get older, the brain changes the way it organizes experience. A year represents 20 percent of a five-year-old's life, but only 2 percent of a fifty-year-old's. The proportion shifts, and with it, the felt weight of each passing year.

The more new experiences you seek out, the more your brain has to work with, and the longer time seems to last.