Garmin displays data in real time. stress level from 0 to 100. Oura calculates”daytime stress” and sustainability indicators. For Whoop, this stress monitor; for Fitbit, it's a “stress management metric.” Whatever it's called, some version of the “stress assessment” has become ubiquitous in smartwatches and wearables. This number is positioned as a window into our inner emotional state, turned into quantitative evidence of how our day is actually going. The only problem: these numbers aren't all that accurate.
What Your Stress Score Really Says
The notes that glow on our wrists don't measure what most of us think they do. When you check your smartwatch and see that your stress levels have skyrocketed, you might assume that the device has somehow detected your anxiety about some direct stimulus, such as a difficult conversation or unpleasant traffic. But this is not entirely accurate.
Of course, your watch may have detected physiological arousal—changes in heart rate variability, skin conductance, or movement patterns. And while these signals do tell us something real about the nervous system, they don't really tell us about stress in the body. psychological feeling like you really care about yourself.
“Part of the discrepancy can be explained by different definitions for conceptualizing stress,” says Eiko Fried, co-author 2025 study who found that stress scores on smartwatches did not match the stress scores reported by most people. The way most people understand the term stress is:I was really stressed today!“is not the way Garmin defines its Stress Score, which measures physiological stress. So your watch doesn't necessarily tell you how stressed you feel, just how your nervous system is behaving. “This increased activity can come from a variety of sources,” Fried says, “including many that we don’t typically think of as stressful experiences.”
Physiological arousal occurs in response to all kinds of experiences that have nothing to do with distress. “What most smartwatches call a ‘stress rating’ is not stress in itself,” says Erwin van den Burghphysiologist specializing in the biology of stress. “This is usually based on indirect physiological signals such as heart rate variability, skin conductance or movement patterns. These signals tell us something about nervous system arousal, but arousal can come from many sources—physical activity, anxiety, caffeine, poor sleep, illness, or emotional involvement—not just psychological stress.”
The oversimplification becomes even more problematic when you consider that most stress algorithms do not take into account sexual physiology, especially the menstrual cycle. Because hormonal fluctuations can significantly change heart rate, heart rate variability, and temperature, “a perfectly healthy physiological shift can be interpreted by wearable devices as ‘high stress,’” he says. Emil RadaitCEO of Samphire Neuroscience. This means that women are more likely to receive misleading stress warnings related to standard human biology, which can be confusing at best and disturbing at worst.
Can you even trust your “stress score”?
Even putting aside the problem of definitions and the issue of gender bias, the fundamental question of measurement accuracy remains.
“If you have heart problems, your cardiologist may ask you to wear a chest strap for a few days to monitor your heart rate and heart rate variability. It’s a high-precision, medical-grade device,” Fried says. “Your doctor won't ask you to wear a smartwatch because there are a lot of problems that make wrist measurements less reliable. This particularly affects heart rate variability, for which we need high-precision measurements.”
What are your thoughts so far?
Heart rate variability is the cornerstone of most smartwatch stress assessments, but wearable devices struggle to measure it with the precision needed to provide medical-grade information. The data isn't useless, but it is noisy, and making clear statements about internal states based on noisy data is, well… scientifically dubious.
So, is your wearable device useless? Of course not. My criticism is not that wearables have no value, but that the value they bring is skewed. Your smartwatch's “stress score” claims to tell you much more than science backs it up. And in some cases, a less-than-ideal outcome may even increase stress rather than help people understand what their body is reacting to. Great The Irony of the Wellness Industry is saved.
Bottom line
The way you think about “stress” does not refer to any one biological state, let alone be expressed in a number or “score.” Your watch is simply detecting signs of arousal in your nervous system, which could mean anything.
This distinction doesn't make the data useless, but it should make you a more informed consumer. It would be nice if companies stopped using the word “stress” to refer to what they're actually measuring – like “physiological arousal” or “autonomic nervous system activity”, which would be more accurate but less marketable, so I'm not holding my breath. (Although if I did, I'm sure my stress levels would skyrocket.)
A device advertised to help you cope with stress may actually increase it by generating anxiety-inducing alerts about normal physiological abnormalities that it misinterprets as distress. The sooner we honestly discuss this gap, the sooner these devices can actually help us, rather than sell us a quantitative illusion of self-knowledge that they don't actually have.
