Etymology: Zeitgeber literally translates from German as “time giver.” The term was coined in the late 1950s by physician Jürgen Aschoff, a pioneer in the study of biological rhythms, or chronobiology. Aschoff established that humans and animals synchronize their circadian rhythms, meaning the cycles that command the body’s internal clock, to the Earth’s rotation. According to his research, our notion of time and the way our bodies adapt to it respond to zeitgebers—environmental time cues—such as sunlight or feeding cycles. In an experiment that lasted over two decades, from 1964 to 1989, Aschoff tested how volunteers responded to being cut off from those zeitgebers when isolated in a bunker for weeks at a time. His conclusion: Devoid of time cues, humans no longer had an ...