Circadian Rhythms (Circadian Rhythms) - 01.18.17

Overview | Description | Applications | Operations | Results | Publications | Imagery

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Science Objectives for Everyone
Circadian Rhythms investigates the role of synchronized circadian rhythms, or the “biological clock,” and how it changes during long-duration spaceflight. Researchers hypothesize that a non-24-hour cycle of light and dark affects crew members’ circadian clocks. The investigation also addresses the effects of reduced physical activity, microgravity and an artificially controlled environment. Changes in body composition and body temperature, which also occur in microgravity, can affect crew members’ circadian rhythms as well. Understanding how these phenomena affect the biological clock will improve performance and health for future crew members.
Science Results for Everyone
Information Pending

The following content was provided by Hanns Christian Gunga, and is maintained in a database by the ISS Program Science Office.
Information provided courtesy of the Erasmus Experiment Archive.
Experiment Details

OpNom: Circadian Rhythms

Principal Investigator(s)
Hanns Christian Gunga, Universitätsmedizin Berlin, Denmark

J. Koch, Denmark
D Kunz
O Opatz
W Schobersberger
A. Stahn
M Steinach
A Werner

Information Pending

Sponsoring Space Agency
European Space Agency (ESA)

Sponsoring Organization
Information Pending

Research Benefits
Earth Benefits, Scientific Discovery, Space Exploration

ISS Expedition Duration
May 2012 - March 2016; March 2016 - February 2017; March 2017 - September 2017

Expeditions Assigned

Previous Missions
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Experiment Description

Research Overview
• The Circadian Rhythms investigation examines the hypothesis that long duration spaceflights significantly affect the synchronization of the circadian rhythms in humans due to changes of a non-24-hour light-dark cycle.
• Synchronization of the circadian rhythm in humans is impacted by a non-24-hour light-dark cycle, reduced physical activity, changes in body composition, and/or changes in body temperature regulation while living in a microgravity environment.
• The outcomes of this investigation might be useful to: (i) understand the time course and basic principles of the adaptations of the human autonomic nervous system in space, (ii) adjust to a more adequate physical exercise schedule and rest-/work shifts, and (iii) foster adequate workplace lighting in the sense of occupational healthcare to humans in space.
• Data on circadian rhythms is collected using a ”double sensor” (core temperature measurement) before, during and post-flight. This data is then correlated with crew members pre and post-flight melatonin (a hormone that follows the classical circadian pattern) levels.


The circadian timing system (CTS) has been shown to be involved in the coordinated daily variation of almost every physiological and psychological system evaluated thus far. Maintaining synchronized circadian rhythms is important to health and well-being. We hypothesize that long-term spaceflights significantly affect the synchronization of the circadian rhythm in humans due to changes in body composition, reduced physical activity and/or changes of heat transfer, thermoregulation, and non-24-hour light-dark cycle in space. Therefore, we aim to investigate the changes of core temperature profiles in humans during long-term spaceflight. Usually, 36-hour rectal temperature profiles are used to determine any changes associated with the CTS. However, such long-lasting continuous rectal temperature recordings are quite inconvenient for the subjects being investigated, especially during daily exercise and hygiene activities. Therefore, we recently introduced the double sensor, a new non-invasive heatflux method for determining body core temperature. The double sensor is located at the forehead and at the sternum/chest and allows continuous body core temperature measurements for extended periods of time.

Data on circadian rhythm obtained with the double sensor pre- in-, and post-flight shall be correlated with melatonin, which is one of the best-studied hormones following a classical circadian pattern. The results derived from the study might be useful to:  first, understand the time course and basic principles of the adaptations of the human autonomic nervous system in space; second, adjust more adequately physical exercise as well as rest- and work shifts; and third, foster adequate workplace illumination in the sense of occupational healthcare to humans in space.

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Space Applications
Circadian Rhythms attempts to understand how the human biological clock adapts to life in space. Future crews could more accurately adjust their sleep, work and physical activity schedules to accommodate natural circadian cycles, which could improve productivity and health. Results may also be compared to the 520-day Mars500 sequestering experiment, and will inform future mission designs for illumination, occupational health, and rest during long-term space missions.

Earth Applications
Maintaining regular circadian timing is important to human health and well-being. Understanding how crew members’ circadian rhythms adapt in microgravity provides a unique comparison for sleep disorders, autonomic nervous system disorders, and shift work-related disorders on Earth. Results from the Circadian Rhythms investigation could also be compared to data collected at the Georg-von-Neumayer Station in Antarctica, where workers also experience abnormal light-dark cycles.

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Operational Requirements and Protocols
• Preflight includes 36 hours of continuous core temperature measurements at the following times: The first two sessions should be planned between L-180 and L-60 days, with a minimum of 30 days between the two data points. A third session is requested at L-30 (±15 days), whereby the requirement for the third session can be waived if the data from the first two BDC sessions is of sufficient quality. A single Bioelectrical Impedance Analysis (BIA) measurement is planned to be performed once pre-flight (L-7, range: L-90 to L-7). It is proposed to conduct the BIA measurement at the same time as the second 36-hrs body temperature measurement.. Urine samples (4 samples per day: evening, morning immediately after waking up, midday (between 10 a.m. and noon), afternoon (between 3 p.m. and 5 p.m.) should be collected as late as possible before flight (range: L-90 to L-2). It is proposed to conduct the urine collection in parallel with second preflight 36-hrs body temperature measurement (preferred, but not a requirement).
• In-Flight data collection includes several sessions which entail 36 hours of continuous core temperature measurements. Data will be collected at FD 15 (±7 days), FD 30 (±7 days), FD 60 (±7 days), and FD 90 (±7 days) for flights of 90 days of duration. For extended flights measurements every 30 (±7) days are desired after FD 90. It is acceptable to reduce the number of inflight sessions to five sessions for a 160 to 180 day mission, as long as the first three sessions (FD 15 (±7 days), FD 30 (±7 days), and FD 60 (±7 days)), one mid-mission session (between FD 90 and FD 120) and one late inflight session (FD 120 to end of mission) can be obtained.During each session, the crew instruments themselves with the Thermolab hardware at the end of Day 1 (1-3 hours before sleep) and wear the equipment for min 36 hours, until the post-sleep period on Day 3. The 36-hour body temperature monitoring is done passively. Physical exercise should be limited to a minimum, if possible.
• Post flight again includes 36 hours of continuous core temperature measurements at the following times: R+7 days (+/-5 days), R+30 days (-7 / +14 days) and R+60 days (-7 / +14 days). Bioelectrical Impedance Analysis (BIA) measurements are to occur on R+7 days (+/-7 days) during routine dual-energy x-ray absorptometry (DEXA) scans by the MedOps facility. Crew member urine collection (24 hours, 4 samples, 4 mL (0.13 oz) to assess melatonin concentrations; sessions to occur as early as possible, but no later than R+7 days.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

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Results Publications

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Ground Based Results Publications

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ISS Patents

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Related Publications

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Related Websites

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image Thermolab Control Unit connected to Portable PFS. Image courtesy of ESA.
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image Thermolab Sensor positions. Image courtesy of ESA.
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image Thermolab sensors. Image courtesy of ESA.
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image Image courtesy of ESA.
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image NASA Image: ISS033E018532 - JAXA astronaut Akihiko Hoshide prepares to perform ultrasound eye imaging.
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image NASA Image: ISS034E052585 - CSA astronaut Chris Hadfield participating in the Circadian Rhythms.
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NASA Image: ISS040E011005 - ESA astronaut Alexander Gerst is wearing a Drager Double Sensor on his forehead which is used on the Circadian Rhythms Experiment.

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