In 1983, NASA flew Neurospora bread mold on STS-9 to test its body clock; the rhythm persisted, but faded out in 25% of flight tubes before light restored it

Well before concerns about jet lag in space became an issue for astronauts, NASA had asked itself the following question: Would a living organism’s circadian rhythm operate in the absence of Earth’s constant 24‑hour rotation? That question framed the STS‑9 Neurospora experiment, as described in a later analysis by Sulzman and colleagues. To answer this fundamental question, NASA decided to experiment by sending cultures of the common bread mold, Neurospora crassa, on board the Space Shuttle Columbia during the STS‑9 mission in November 1983, as outlined in NASA’s summary of the STS‑9 mission. It turned out that the rhythm persisted in space overall, though it faded in about a quarter of the cultures until regular lighting restored it, according to the flight results reported by the same research team.

Why a common bread mold was the right test subject

The fungus Neurospora crassa was selected for this experiment because it has a very distinctive circadian rhythm, which is easily observable to the unaided eye. As it grows in a narrow glass tube, it progresses in a uniform front, producing orange spores called conidia at intervals of about 22 hours, resulting in alternating bands of color inside the tube, as described in reviews of Neurospora circadian rhythms. This phenomenon had been studied for decades through what is referred to as “race tubes” to study biological rhythms on Earth; NASA got an extremely easy way to measure the rhythm of this organism without having to employ any complex instruments or relying on behavior. The big question was, did the rhythm occur independently or was it dependent in some way on a geophysical cue related to the rotation of the Earth?


A ten-day flight carrying an unusual cargo

Columbia launched from Kennedy Space Center on November 28, 1983, carrying the European Space Agency's Spacelab module on its maiden voyage, a 10‑day mission crewed by six astronauts, the largest crew flown on a single spacecraft up to that time, as confirmed by NASA's and ESA's mission summaries. As recorded by NASA on its official website, Spacelab 1 was a mission comprising numerous experiments including those of astronomy, materials science, and life sciences, with Neurospora being one of them.

A rhythm that faltered, then recovered with light

In this case, the scientists J.S. Ferraro, C.A. Fuller, and F.M. Sulzman analyzed the banding rhythm in flight and reported their results in the paper entitled The Biological Clock of Neurospora in a Microgravity Environment, published in Advances in Space Research (1989), building on Sulzman and colleagues' earlier 1984 report in Science, which had first raised the same question. Over the first seven to eight cycles in orbit, the rhythm persisted but became ragged: the period between bands lengthened slightly, growth rates became less constant than usual, and the amplitude of the rhythm decreased. At the same time, in one quarter of the tubes, the rhythm dampened and did not appear to band on any rhythm at all. After that, however, on day seven, a standard operation involving light exposure revived the rhythm and made it strong and steady for the rest of the flight.

Because the fading occurred only in the days before that light exposure, and not after it, Ferraro, Fuller, and Sulzman, writing in their 1989 Advances in Space Researchanalysis, hypothesized that the effect had nothing to do with microgravity itself but instead resulted from the hypergravity pulse of launch, though they cautioned that their data could not definitively distinguish an effect on the clock mechanism from an effect on how that mechanism was expressed.

What the surviving rhythm revealed about the clock itself

In fact, it was Sulzman et al.'s original 1984 paper in Science, published shortly after the flight, well before Ferraro, Fuller, and Sulzman's more detailed 1989 analysis, that first explored what the flight data meant for the broader question of the origin of circadian rhythms. It was revealed that the free-running period of the fungal rhythm during space flight was virtually identical to the one demonstrated by the same strain in earthly conditions, supporting the idea that the clock is not just an adaptive response to any daily geophysical signal associated with the rotation of the Earth. However, as already pointed out in the above publication, the researchers were reluctant to make a final claim as to whether the observed irregularity of the initial phase of the rhythm was actually caused by an actual disruption of the clock process or merely a malfunction of its manifestation.

Why this small fungus experiment still gets cited

The STS-9 Neurospora experiment is an early published example of how a circadian clock behaves in spaceflight. Long-duration spaceflight is known to disrupt astronauts’ circadian rhythms, and this simple fungal experiment provided early evidence that a biological clock could continue functioning aboard a spacecraft, even if launch stresses or the absence of Earth’s usual day-night cycle temporarily altered how that rhythm was expressed.