Einstein determined -- and it was proven -- long ago that clocks in space move slower than clocks on Earth because time moves at different rates depending on how close an object is to a gravitational field. In a new study published today in Science, researchers used atomic clocks accurate to within one second in 3.7 billion years to show that gravitational fields also affect time on a much smaller scale. Basically, your head ages faster than your feet -- unless you spend most of your time hanging upside down.

In one experiment, James Chin-Wen Chou and his colleagues placed one clock about 13 inches higher than its counterpart. The higher clock felt less gravity, because it was a teeny bit farther from Earth's gravitational field. It ticked more slowly -- albeit a tiny, tiny bit more slowly. The time difference adds up to about 90 billionths of a second over a 79-year lifetime, according to NIST.

Still, this means that the people who conducted this study, in Boulder, Colo., are apparently aging faster than those of you reading this at sea level.

In another experiment, the NIST scientists also observed that time passes more slowly when you move more quickly -- a key tenet of relativity -- even at very small speed variations. Clocks ticked more slowly at a difference of just 20 miles per hour, they say.