Micro-fluctuations in the time between heartbeats are proving a helpful indicator of mental health, stress levels, exercise capacity and even how well you are ageing.
Artem Kirillov is not, by nature, the kind of person to take it easy. "I prefer to push myself further, even if I feel a bit off," says Kirillov, who is 40, lives in London and works in health tech. … [+201 chars]