zeitgeber
 n.— «Light and temperature are the only periodic or quasi-periodic environmental variables to which endogenous oscillation can be coupled: in nature they entrain the endogenous oscillation, thereby controlling period and establishing appropriate phase. They are, to use Aschoff’s phrase, the principal Zeitgeber.» —“On the Significance of Transients in Daily Rhythms” by Colin Pittendrigh, Victor Bruce, Peter Kaus Proceedings of the Natl. Acad. of Sciences (US) Sept. 15, 1958. (source: Double-Tongued Dictionary)