Not getting enough sun? That might not be as bad as you think. Although too little sunlight exposure has been tied to exhaustion, vitamin D deficiency, and seasonal depression, new research has shown that too much sun can reduce human fertility over several generations.

That's at least according to a study recently published in the journal Proceedings of the Royal Society B, which details how researchers studied 150 years of birth, marriage, and mortality data encompassing more than 9,000 people that starts as church records in 1750.

Accounting for environmental factors of where each subject lived, researchers found that the average lifespan of children born in years that had a great deal of solar activity was 5.2 years shorter than other children. This, they said, was heavily influenced by early life mortality - that is, the rate of child deaths before the age of 2.

Interestingly, the records revealed that children who survived to adulthood in years with lots of sunshine were also more likely to have fewer children, and those resulting children were in turn less productive than mothers from less sunny generations.

This was certainly surprising, as more sunlight exposure is notably associated with better health and a better agricultural year, reducing the chances that starvation or malnutrition were key influences.

So what's going on here? The study only pointed out an unusual association, and did not investigate cause-and-effect. However, researcher Gine Roll Skjærvø at the Norwegian University of Science and Technology suggests that ultraviolet radiation's effect on vitamin B9 (folate), may have a part to play.

Low folate levels during pregnancy have previously been linked to higher child mortality, and UV radiation has been known to reduce uptake of this vitamin in humans. However, it important to note that low levels of this vitamin are not uncommon even today, and how exactly this can impact multiple generations remains a mystery.

"There are probably many factors that come into play, but we have measured a long-term effect over generations," Skjærvø said in a statement.

"The conclusion of our study is that you should not sunbathe if you are pregnant and want to have a lot of grandchildren," the researcher added, taking a 'better to be safe than sorry' stance on the subject.

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