Based on plenty of chats with runners I've worked with where we've discussed the impact of stressful situations e.g. family problems, difficult times at work etc on running I've been doing some thinking and a little research!
Humans aren't build for long periods of stress, our brains respond really well to short term stress (like the appearance of a bear!) with a hormonal response that prioritises an immediate response (fight, flight or freeze). The immediate response mode is trigger by the release of two key chemicals; adrenaline and cortisol. They help raise our heart rate, prepare our muscles etc to react to the situation. They also turn down other systems in the body that won't help out immediate response; immune system, digestion, reproduction etc.
This is a great system, but it becomes an issue when stress is a longer term thing; for example a highly stressful family issue or working for a dysfunctional and unsupportive company. Longer term stress levels also have the same chemical response in the bodies, but this will have negative effects on our bodies and brains. When we don't have enough calm rest time we won't process our foods well, have normal hormonal function, or recover as fast from training sessions. It can even slow down our recovery from injury.
Often avoiding these longer term stressors is difficult, especially if it's your job... However finding ways of prioritising down time, mindfulness and physically inactive mentally calming activities can help.
Below are a few links on the science
The links between “psychosocial” things like stress and the emotional support you have to injury healing, surgery recovery etc; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3052954/
Impact of longer term stressful situations on sports performance
Mindfulness improving sports performance
