Human physiology is a window to our physical, mental, and emotional states; our well-being. Today, a new wave of objective data derived from consumer grade body sensors-like those equipped by smartwatches-paves the way toward a new approach in how well-being is being measured, continuously and unobtrusively. Here, we developed a framework for collecting and analyzing physiological data using smartwatches in-the-wild, and demonstrated its robustness in data obtained away from controlled laboratory settings. We found that changes in people's heart rate and heart rate variability are predictive not of momentary well-being (a scientific idea that continues to live on in the absence of in-the-wild evidence, aka, zombie theory) but of daily well-being.