Stress is part of life. A definition from the internet says it is the response to a stressor (probably a circular logic). A stressor can be anything that scares you, your boss, a real lion in front of you (not the tv/zoo lion that you are imagining). It is very important that we keep stress under control as it can have really bad side effects including cancer (don’t get stressed about this! Need a lot more studies).
How can you control something which you cant measure? I have done a recent work that uses a detailed model (dirty equations) of our blood pressure regulation system and radial arteries with the aid of machine learning to measure stress from the PPG (photoplethysmogram). Sorry for the long sentence – use math to predict stress from PPG. We can even get PPG signals using mobile camera – therefore measure stress :).