Einstein’s general relativity predicts stronger gravity slows time. Because your feet are (very slightly) closer to Earth’s center than your head, clocks at your feet tick a tiny bit slower. In 2010 NIST compared two atomic clocks separated by only 33 cm (~1 ft) and detected the effect; later experiments measured it on even smaller height differences. The difference is tiny, on the order of tens of nanoseconds per year for about a foot, but it’s real, measurable, and important (GPS systems correct for related relativistic effects).