exogenous zeitgebers- campbell and murphy (1998) conducted an experiment where pps had woken in the middle of the night, after waking, one group had light shone on to the back of their knees whereas the control group with the same procedure went through the same procedure but with an unplugged lightbulb, researchers observed that the group who had light shone on their rear knees had a greater deviation from their original circadian rhythm compared to the control group. This demonstrates the importance of exogenous zeitgebers of light even when not in contact with the eyes but the skin for regulating a circadian rhythm
