The term "zeitgeber" (German pronunciation: [ˈtsaɪtˌɡeːbɐ], literally "time giver", i.e., "synchronizer") was first used by Jürgen Aschoff, one of the founders of the field of chronobiology. His work demonstrated the existence of endogenous (internal) biological clocks, which synchronize biological rhythms. In addition, he found that certain exogenous (external) cues, which he called zeitgeber, influence the timing of these internal clocks.