Bunker experiment

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] It is now known that these clocks operate based on molecular oscillations, using transcription-translation feedback loops and metabolic feedback loops.[7] External cues, notably light, interact with this clock by entraining it to match the external time.[7] However, before this experiment, some researchers speculated that human rhythms were driven by these external cues without influence from an internal clock.[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.[8] The bunker contained a bedroom, kitchen, and shower, but no windows, and many participants were students who used the time underground to study.[3] Many participants found the idea of living in a bunker without time cues daunting, but most ended up enjoying the experiment.[1] Aschoff was the first test subject, with Rütger Wever observing.[2] The experiment began in 1966 and continued until the early 1980s.[1] As of 2013, the bunker was still in existence, although quite dilapidated.[9]

The inside of the bunker was isolated from all time and light cues and subjects lived there for weeks at a time.[1][3] Subjects prepared their own meals and were asked to eat three meals a day, not nap after lunch, and perform some psychological tasks.[3] Besides this, they could generally do as they pleased, and were able to turn the lights on and off themselves.[3] The bunker was separated from the outside world via two doors, only one of which could be opened at a time. Researchers would place food in the space between the doors, and the subject would leave urine samples here. The experimenters made deliveries at randomized times. The subject could communicate with the outside only through letters.[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] They also examined whether male and female clocks function differently.[2] Many measurements were taken on subjects including their sleep-wake activity, bed movements, rectal temperature, urine samples, and time perception. To measure time perception, individuals were asked to press a buzzer at one hour intervals and a minute after the one hour mark.[1][3]

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.[7]

The demonstration of the human circadian clock led to research that uncovered many of the mechanisms that underlie it.[13] Desynchronization observed in this experiment was replicated by Aschoff and Wever in later experiments and used as evidence for a multi-oscillator model of circadian rhythm control.[11][14] Scientists also studied the genetic basis of circadian rhythms. Ronald Konopka and Seymour Benzer discovered the period gene in 1971, which disrupted these rhythms when mutated in fruit flies.[5] Jeffrey Hall, Michael Rosbash, and Michael Young uncovered the underlying cellular mechanism by which the period gene functions, research for which they received the Nobel Prize in Physiology or Medicine in 2017.[15]

Many scientists have taken inspiration from the bunker experiment, performing a myriad of studies on the human circadian clock. 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.[2]

• Max Planck Society website