生物钟 - shēngwùzhōng - biological clock

Why time seems to speed up the older we get, according to science

It's one of life's great injustices: The older you get, the more time seems to fly. If you find yourself frequently wondering where the time's gone as you age, you aren't alone. It turns out, it's a real perceptual phenomenon that most people experience.

Of course, time doesn't really speed up; clocks everywhere aren't ticking any faster than they were when you were a child. But the passage of time is also a perception, and there's evidence that the pace of time, as a perception, really does accelerate, reports Discover Magazine.

The simplest explanation for why time seems to speed up is that our brains calculate the perception of time roughly based upon the percentage of time we've lived up to a particular point. For instance, when you're a 2-year-old, a year represents half of your life. Looking back on your early years, it may seem like a year took a lifetime — or at least, half a lifetime. That's because it really did. But a year becomes a smaller percentage of your life with every flip of the calendar.

By this measure, when you're a 10-year-old, a year is only 10 percent of your life. By age 20, it's only 5 percent, and so on. To put things into dismal perspective, this means the period of time spent between ages 10 to 20, a 10-year period, is perceived to elapse as quickly as time did from ages 5 to 10, just a 5-year-period. Obviously it only gets worse as you age. These periods are equivalent to the time it seems to take to get from age 40 to 80. That's 40 years that seems to fly by at the speed of just five younger years.

That's your biological clock ticking

But why does your brain measure time in such a seemingly cruel way? There's more to it than just a logarithmic calculation. Our bodies also have a number of biological clocks, and these clocks tend to slow down as we age. For instance, our metabolism slows, which can lower the pace of our heartbeat and breathing. If you think of your breaths or heartbeats like ticks on a clock, then when you're younger there are more ticks in a minute than when you're older.

This effect might also partly explain why time can occasionally seem to slow down when we're exerting ourselves. Thrill-seekers report a similar slowing-down of time as their adrenaline levels spike, which also quickens their hearts and breaths. It might also be what's going on when people claim to see their entire lives "flash" in front of their eyes in a death-defying moment.

Yet another factor that likely plays a role in our perception of time is the release of dopamine in our brains when we're processing novel experiences. When we're younger, more of our experiences are novel; you experience a lot of firsts when you're a kid. Novel experiences require more brain energy to process, as well as more time. So in our younger years, our brains are more hyperactive, taking in every detail, as they work to understand the world. Once we figure out how things work, however, fewer details need to be considered. Our brains might slow down, but this also makes time seem to speed up, since fewer and fewer moments tend to matter as much.

Fountain of youth?

All of this seems to offer a solution, however. Perhaps there's a way to beat the accelerating rate of time. If you stay curious and active and continually seek out novel experiences, maybe you can even the odds; maybe you can slow time down again, like when you were young. Maybe curiosity and new experiences are the true Fountain of Youth.

It's an interesting twist on the old motto, that you should live every moment like it's your last. Rather, maybe the secret is in living every moment like it's your first.

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