A decade ago, a wave of companies promised to transform people’s health by allowing them to track data about their eating, sleep, exercise, and other habits. One hot startup of that moment, Zeo, raised more than $30 million from investors to develop a headband that tracked users’ sleep patterns and an accompanying app to serve as their personal “sleep coach.” Despite devoted users and buzz about its product in publications like Wired and Popular Science, Zeo quietly went out of business a few years later.