Yang et al. They suggest that although light can entrain a behavioral rhythm, the eyes are not a required medium for the light to cause a shift in a rhythm. This also lends credence for the theory that circadian rhythms and their components being wide spread throughout the body and occur at both the cellular and subcellular levels. An entraining agent is that which synchronizes the internal clock to environmental cues. Environmental cues are external cues, which resets the circadian clock.
Light is a very common cue used by circadian pacemakers to keep time.
Environmental cues, entraining agents, and zeitgebers are interchangeable terms. A free-running rhythm is a circadian rhythm that is independent of environmental cues. An infardian rhythm is a cycle that is greater than 24 hours. An example would be the frequency of receptivity of female fire flies to male fire flies for mating. In vitro refers to occurring outside the living body, but within an artificial environment.