"<i>Zeitgeber</i> (from German for "time giver," or "synchronizer") is any exogenous (external) cue that synchronizes an organism's endogenous (internal) time-keeping system (clock) to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light. Non-photic zeitgebers include temperature, social interactions, pharmacological manipulation and eating/drinking patterns. To maintain clock-environment synchrony, zeitgebers induce changes in the concentrations of the molecular components of the clock to levels consistent with the appropriate stage in the 24-hour cycle, a process termed entrainment."