Astronauts’ Circadian Rhythm On International Space Station Surprises Scientists

ESA astronaut Samantha Cristoforetti participates in the Circadian Rhythms experiment during her mission on the International Space Station in 2014–15, in this photograph released June 8, 2017. Photo: NASA/ESA

Our bodies have an internal clock, called the circadian rhythm, which is in sync with the 24-hour cycle of day and night. It is regulated by the body’s core temperature and is linked to sunlight, insofar as the core temperature is affected by sunlight. The rhythm itself triggers important bodily functions like metabolism and the sleep cycle.

Given the lack of sunlight in space, scientists predicted that astronauts aboard the International Space Station would find their circadian rhythms disrupted. But measurements of 10 astronauts’ core temperatures have surprised scientists, leading them to say “you can take the body out of Earth, you can’t take an Earth-based rhythm out of the body.”

The core temperature of the human body, when on Earth, is 37 degrees Celsius (98.6 degrees Fahrenheit). It decreases by half a degree early mornings and goes up by half a degree in the early evening. An experiment sponsored by the European Space Agency is looking at how this change varies in space.

“If our bodies are an orchestra, core body temperature is the conductor, signaling when hormones and other systemic functions should come into play,” Hanns-Christian Gunga of the University of Berlin, principle investigator of the experiment, explained the role of the core temperature in a statement Thursday.

The disruption of the circadian rhythm predicted by researchers theorized that the core temperatures of astronauts’ bodies would drop. Instead, measurements showed core body temperature increased overall. Also, the half-degree change in the temperature twice in 24 hours slowly shifted by two hours. This shows the body works harder and is, therefore, warmer to keep the circadian rhythm going. Researchers working on the experiment are not yet sure of why the shift in the triggers for bodily actions like metabolism and sleep occurs.