Apps like this use data from your phone's accelerometer to measure how much you move during your sleep. This is effective at guessing sleep duration and, well, how much you move during your sleep. That's about it. There are devices that measure additional things like heart rate variability, breathing, etc., but even these are limited in their accuracy of detecting sleep phases and quality of sleep. It seems to me that these apps are only good for keeping you thinking about your quality of sleep, not actually tracking it. Quote from Patrick M. Fuller, assistant professor of neurology at Harvard Medical School: “The concept of taking information from your iPhone and applying it to sleep cycles is cute. The designers are well-intentioned and conceptually it’s neat, but the catch is that the utility of information derived from a phone tucked under your pillow is really limited.” Quote from Paul Kelley, a clinical research associate in the Sleep and Circadian Neuroscience Institute at the University of Oxford: "The data that comes out is probably in the right ballpark about duration.... It may be less clear about quality of sleep." Quote from Colin Shapiro, unit program director at Toronto Western Hospital's Sleep Research Unit: "We're still a long way off from getting a full idea of sleep from these wearables."

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