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plural zeitgebers or zeitgeber
(physiology) A rhythmically occurring cue given by the environment, such as a change in light or temperature, to reset the internal body clock. quotations
Thus, while light is the most reliable and thus the primary zeitgeber, there are many factors that can be used in addition to, or in the absence of, daylight.
2017, Matthew Walker, Why We Sleep, Penguin, published 2018, page 18