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What Devices Read
Heart rate, heart rate variability, and a few other signals are what today's devices use to tell you how stressed you are, how recovered you are, and how well you are sleeping. None of that is a direct readout from the autonomic nervous system. The usual assumption is that every rise in heart rate and every drop in HRV is bad.
A Direct Cuff
A neuroscientist who did her undergraduate work at Stanford and her graduate work at Harvard has developed a small cuff that measures activity from the autonomic nervous system directly. At the end of the day, it reports how different activities and patterns of speech and breathing related to distress versus eustress. Her data show that certain parts of a day, like being stressed while doing things you love, set up better sleep and more focus.
Stress Tails and Focus
The cuff can also show whether a person is truly stressed or simply in a high arousal state, and how long the tail of stress lasts after a stressful interaction. She has brought it to police departments and first responders. Focus would probably be read from changes in pupil size, the way it is done in animals, which points to eyeglasses.
Why can heart rate variability mislead?
Heart rate variability reflects the push and pull between the sympathetic and parasympathetic branches, but it is also shaped by breathing rate, posture, caffeine, illness, and recent exercise. Two people with the same number can be in very different states. Because the heart's rhythm is only an indirect reflection of the autonomic nervous system, researchers have long looked for direct measures. Recording from the nerves themselves, a technique called microneurography, has been used in labs since the 1960s.
Where did the idea of good stress come from?
Hans Selye, the Hungarian-Canadian endocrinologist who popularized the concept of stress in the 1930s, introduced the term eustress in 1976. He used it for the kind of stress that accompanies challenge and fulfillment, as opposed to distress. Later research has found that how people appraise a challenge changes their physiological response to it. Selye's work earned him multiple Nobel Prize nominations.
How does pupil size track focus?
Pupil diameter rises with mental effort, a link Daniel Kahneman documented in the 1960s when he watched participants' pupils widen during mental arithmetic. The pupil's size is tied to the locus coeruleus, a brain stem structure that releases norepinephrine and regulates arousal. Because of that link, eye trackers are used in research as a read-out of attention. The pupils widen and narrow in milliseconds as the demand changes.
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