Working out what makes you stressed and how much is too much can feel quite subjective. Increasingly, however, technology can help.

Most smartwatches can give you a basic reading of stress using your heart rate. A healthy resting heart rate for an adult is between 60 and 100 beats per minute. Cortisol and adrenaline, released when your stress response kicks in, can raise this; a poor ability to recover from stress can keep it raised.

Many smartwatches also track heart rate variability (HRV), a measure of the natural variations in time between consecutive heartbeats. When your body is stressed, cortisol and adrenaline trigger a fast and consistent heart rate, reducing this natural variability between beats. When the parasympathetic system kicks in to restore balance, your natural variation increases. Average HRV varies between people, so it is best to use deviations as a way of monitoring your stress levels.

Over time, heart rate and HRV can be used to give you a stress “score”, helping you identify certain activities, people or times of the year that cause you too little, or too much, stress (see “Why the right kind of stress is crucial for your health and happiness”). However, this is a blunt tool, with a study last year showing that such stress scores can’t distinguish positive stress (or excitement) from the negative kind.

Cortisol is another biomarker of interest to stress researchers. But it isn’t ideal because it spikes around 20 minutes after a stressor occurs, says Julie Vašků at Masaryk University in the Czech Republic, and involves taking a saliva, urine or blood sample that is analysed in a lab. Biosensors that sit in the arm and continuously monitor cortisol in blood plasma are in development, but aren’t commercially available.

In the future, we might be looking to our bones instead, says Vašků. When you are stressed, bone cells hoover up a substance in the blood called glutamate, which normally switches off production of a hormone called osteocalcin.

This causes osteocalcin to flood the body, dialling down the parasympathetic nervous system and allowing the fight-or-flight response to proceed.

“We think, under stress, the skeleton produces a lot of molecules, very quickly, that are actually better biomarkers of what is happening at the time,” says Vašků.

“These bone-derived molecules are helping direct energy where it needs to be,” she says. “In the future, we think one of these molecules could be a really good biomarker for stress.”

This article is part of an in-depth look at stress. Read more here:
Why the right kind of stress is crucial for your health and happiness
Can we ‘vaccinate’ ourselves against stress?