[4] Before this experiment, scientists had already discovered that many plant[5] and animal[6] species have intrinsic clocks that can operate independently of external time cues.
[1] To test the hypothesis that humans have an internal clock, Jürgen Aschoff and colleagues had a bunker of reinforced steel[1] built in Andechs, Upper Bavaria, Germany.
[3] Over about 20 years, the researchers studied various properties of the human biological clock, varying social interaction, light intensity, gong sounds, or electromagnetic radiation to test the impacts on circadian function.
[2] Many measurements were taken on subjects including their sleep-wake activity, bed movements, rectal temperature, urine samples, and time perception.
[1][3] The bunker experiment conducted by Aschoff demonstrated that the participants' urine excretion, rectal temperature, and circadian rhythm of activity all had a free running period of approximately 25 hours in the absence of external cues.
[12] This experiment provided evidence that humans have a circadian clock that can run independently from the 24-hour light-dark cycle and that different people can have unique rhythms.
Maroli Krishnayya Chandrashekaran and Geetha L. replicated Aschoff's findings in 1996 and also showed that the menstrual cycle in a human female in isolation was not coupled to the sleep-wake rhythm.
[16][17] A 2020 case study by Maria-Angeles Bonmati-Carrion and colleagues used a similar bunker protocol to study the human circadian clock in social and environmental isolation under light-dark conditions and in constant dim light, again finding free running periods longer than 24 hours in constant dim light.
[4] The bunker experiment had far-reaching implications in the medical field, for example in understanding problems related to shift work and jet lag.
[18] In addition, it allowed for a better understanding of affective disorders in relation to the synchronization of the internal circadian clock and the light-dark and sleep-wake cycles.