The question of biological time requires us to imagine our bodies in the context of their environments. While we all carry within us an internal clock (or circadian timing system) which has evolved over millennia, our clocks are also constantly in conversation with our surroundings. The daylight, the glow of our screens, our meals, and even our social interactions all work to keep our biological time in sync with the world around us. This overlay between the so-called endogenous and exogenous factors of our body clocks makes them very hard to study. That is, unless scientists can find a place that lies ‘beyond time’. Today, scientists have highly specialized labs that help them to do this. But in the past, chronobiologists had to find unique timeless spaces to carry out their experiments.

Ask a chronobiologists about the history of the discipline, and you find that specific places are central to it. The Andechs bunker is perhaps the most famous of these ‘timeless spaces’ – a facility built for interrogating the human biological clock in a disused World War II bunker by German scientists Rütger Wever and Jürgen Aschoff. From the late 1960s through the 1980s, the bunker was a central site for understanding the body clock isolated from the outside world. Before the development of specialized temporal isolation facilities, early chronobiologists often sought out unique natural places in which to carry out their experiments. Experiments taking place in nature (often known as field experiments) capitalize on unique naturally occurring conditions that might be difficult or too expensive to recreate in a laboratory. These ‘timeless spaces’ were perceived to have a unique ability to remove human subjects from the influences of society and the natural world in order to understand what lay beneath the body’s timing system. For early circadian scientists, it was crucial to create situations in which human participants would ‘free run’ – untethering their rhythms from the clock – in order to examine which rhythms were intrinsic to the body. I want to think about just one of these here: the cave.

Sleep scientist Nathaniel Kleitman takes readings of Bruce Richardson as he sleeps in Mammoth Cave, Kentucky, 1938. Copyright: Open Parks Network.

‘Mr Richardson and I have now been in Mammoth Cave for a little over four weeks. We have come here to make observations on the physiology of sleep as a part of a study that has been going on at the University of Chicago since 1922. We were entirely successful in our undertaking and we want it known that it is in no way a stunt or an act of endurance or perseverance, but a bona fide scientific experiment.’ – Nathaniel Kleitman, News of the World, 1938

Speaking to the camera of the American news programme News of the World in 1938, physiologist and sleep researcher Nathaniel Kleitman (1895–1999) declared his 32-day Mammoth Cave experiment a success. Designed to see if it was possible to adapt the human body to a 28-hour day, the experiment was a part of a much longer research interest for Kleitman in the environmental and cultural elements of sleep. In this quote, Kleitman hints at his anxiety about the perception of his field work as bona fide – both as a result of the mundane nature of sleep itself, and of the location of the research, a world away from the laboratory.

In the 20th century, the cave became a paradigmatic place to do circadian science. Removed from the world and the dark-light cycle of the Earth, a cave seemed to provide an ideal isolated environment in which to analyse the human clock. There is something about caves that feels primordial – humans evolved to live in caves, so perhaps we might return there to unravel the mysteries of our evolution. In 1938, Kleitman took to a cave in Kentucky for a month along with one of his graduate students to see if they could adapt their biological rhythms to a 28-hour day. Armed with an alarm clock, Kleitman and Richardson attempted to force their bodies onto a six-day week, tracking their body temperatures and sleep as a way to confirm whether such a shift had occurred. In the end, Richardson was able to change his rhythms though Kleitman could not – a fact that Kleitman attributed to age. Trying to force the body off of one timing routine and on to a new one is known as a ‘forced desynchrony protocol’ – and it still in use (although with much more sophisticated equipment) by chronobiologists today.

Kleitman’s was the first and most famous of these cave experiments, but far from the only one. In the mid-1960s, Manchester-based physiologist John Mills (d. 1977) carried out a series of cave isolation experiments, each involving a single volunteer – the first living in Stump Cross Caves in Yorkshire for three months, and a later subject staying in Cheddar Gorge for four months. In the Stump Cross cave, Mills separated the role of researcher and participant– with researchers from the unit visiting the cave-dwelling participant to collect blood and urine samples as well as measuring temperature. Protocols for avoiding sharing the time with the participant were far from sophisticated, although researchers took care to attend to their facial hair to make sure a 5-clock shadow give away the time of day. These experiences later informed the construction of Manchester Department of Physiology’s own time isolation bunker built at a nuclear power station.

Michel Siffre collapses after two months underground, 1962. Image from Beyond Time (London: Chatto and Windus, 1965).

Much more famously, the early 1960s saw French speleologist Michel Siffre (b. 1939) descend into a cave in southwestern France for a period of two months in complete isolation – save for a telephone which only operated in one direction. As Siffre was not a physiologist by training, his interest in ‘nycthemeral’ rhythms ( a common term at the time) was arrived at from a very different direction. Siffre was interested in how the cave might be an analogue for space particularly in the early space age of the 1960s. Siffre hypothesized that mental fortitude in isolated conditions was even more important than physical preparations. Inspired by extreme adventurers – Siffre resolved to experience life outside of time.

Man’s faculty of adaptation to conditions absolutely different from any known on the earth’s surface is more and more being demonstrated, and found to be almost unlimited. This adaptive power is important in an epoch when the domestication of the atom with its consequent powers of destruction may oblige mankind to take refuge in underground shelters. It is also important to the dawning age of space travel, when individuals will be obliged to adapt themselves to a changeless environment, deprived of any points of reference in time and space. – Siffre ‘Beyond Time’ 1965 p. 19

He undertook physiological tests before and after this experiment, although while in the cave focused mainly on geological and speleological work. Like Mill’s participant, he was successful in ‘free running’ – desychronizing his clock from the outside world and losing track of time. When he was pulled out from the experiment on 14th September 1962, he was convinced it was only 20th August – losing track of several weeks! Despite this, his bodily rhythms still ran on a roughly 24-hour cycle. Siffre’s continued his cave experiments throughout the 20th century – attracting considerable media attention.

CAVES Basecamp in 2011, European Space Agency (ESA)

Today, the cave is still an interesting place for scientific investigation. Most circadian scientists would say that caves are not ideal work places because of the harsh conditions – they are dark, damp and uncomfortable for both researchers and research subjects, especially in comparison with specialized labs. Scientists working the middle part of the 20th century were also much more interested in what happens when people were isolated from ‘society’ – and less in the effects of light deprivation, which we now know to be an important part of setting the clock. But caves still hold an important place for groups who are hoping to model isolation conditions – especially in the context of space exploration. The extent to which human circadian rhythms are adaptable is still a key concern for space agencies. Since 2011, the European Space Agencies has used caves as a part of its astronaut training programme. The CAVES initiative (Cooperative Adventuring for Valuing and Exercising human behaviour and performance Skills) aims to give astronauts experience in working in multicultural teams in challenging subterranean environments. But as researchers have recently argued – there is much about the complex cave environment and its physiological effects that we don’t fully understand and are difficult to recreate in a lab.[1] The cave seems likely to be a scientific ‘space’ of the future, not just the past!


[1] Zuccarelli et al. (2019) ‘Human Physiology During Exposure to the Cave Environment: A Systematic Review with Implications for Aerospace Medicine,’ Frontiers in Physiology, 10:442.

This research is a part of the Body Time project. A version of this work by Kristin D. Hussey has been accepted for presentation at the European Society for the History of Science Conference 2022 in Brussels

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