In 2015, Nasa sent Scott Kelly to space and left Mark Kelly, his twin brother, on Earth. The experiment helped them determine that, due to the lack of stress on the skeleton, a person loses ~1% of their bone density after one month in space. For comparison, an average person aged 65 and older loses that 1% over the course of a year.

Positive stress can be referred to as eustress, while negative stress can be referred to as distress. However, things are not quite as black-and-white as they seem.

When we are faced with stress, we may either react with a threat response, or with a challenge response. The first one occurs when in danger, such as when being chased by a predator, while the latter occurs when presented with a difficult task, such as when chasing prey.

While the threat response can be detrimental to your body, the challenge response has been shown to be beneficial, improving blood flow to the brain and even the ability to learn.