It is well known that there are a number of negative health issues (loss of bone density, muscle mass, (eyesight degeneration1, etc.) that can impair astronauts who spend long periods of time in space. That said, are there no positive effects on the human body at all?
Edit 1: To address the comments, let's assume that the astronauts are eventually going to return to Earth.
Edit 2: ... And hope to do it alive.
1See What are the effects of body building on heavy planets?
There have been no known long-term positive health effects reported.
Then again, euphoria and elation are considered negative health effects, and both have been reported, and many other causes are used as recreational stress relief.
Almost all astronauts report positive mental attitude changes from their time in orbit. This isn't exactly measurable, tho', and I've not seen any well done studies done on Astronauts who flew versus those who didn't on various psychological effects. †
Short term, ISTR reading that blood pressure generally drops. And there is a study reporting that blood pressure and heart rate drop while in microgravity. ①. This does, however, appear to be maladaptive for return, as it also is accompanied by cardiac atrophy.
This reduction could be of use for certain individuals at grave risk of stroke or heart attack from hypertension; the reduction in blood pressure and heart rate is known to be a direct reduction in stroke and heart attack risks. The problem being that current launch protocols involve a highly stressful period of increased workload for launch, which is counter-indicated by the very condition which would benefit from the reduction. ‡
Certain forms of depression might benefit from spaceflight and microgravity. The profound "life changing experience" reported by almost all astronauts may be of benefit, but until it is more affordable, it's unlikely to be tested.
① Microgravity decreases heart rate and arterial pressure in humans.www.ncbi.nlm.nih.gov/pubmed/8964756
† The problem being that doing the study is laden with confounding factors and a lack of a viable control group. You'd need a control group who were selectable for astronaut training, but told from the outset they are never going to get to go... Which would be an interesting study in and of itself, but would be impractical financially, might be unethical, and certainly still wouldn't be entirely free of confounding factors.
‡ If some form of gravitic lift is developed, then it's quite likely that this could be of use; it's equally likely that any such lift would render spaceflight unneeded for the same benefit, as any such system would likely be able to generate local microgravity conditions.
It theoretically could. There is a hypothesis of radiation hormesis which states that:
[...] low doses of ionizing radiation (within the region of and just above natural background levels) are beneficial, stimulating the activation of repair mechanisms that protect against disease, that are not activated in absence of ionizing radiation.
If we assume this hypothesis to be true, and have in mind that the flux of ionizing radiation increases with altitude, a logical reasoning would result in the conclusion that a short space flight, exposing astronomers to a low dose of ionizing radiation, could have beneficial effects. However, depending on the flight's altitude, elevated radiation exposure could be achieved by any sub-space flight, provided that its duration was sufficiently long. Alternatively, there are many different ways of exposing oneself to elevated flux of ionizing radiation which do not involve participating in any flight at all. Therefore, this supposed positive effect isn't exclusively specific to space flights, and it is arguable whether it could be considered a good example.
