Sleep-tracking devices have limits. Experts want users to know what they are

Millions of people rely on wearable devices to monitor how well they're sleeping. Sleep trackers don’t necessarily measure sleep directly but instead infer states of slumber from signals like heart rate and movement. Experts say it's therefore important to understand what the devices can and cannot measure. A University of Michigan math professor who researches the science behind sleep wearables says the algorithms used by major brands are highly accurate in determining when someone is asleep and somewhat helpful for estimating sleep stages. But a neurologist at the Morehouse School of Medicine says some wearers become fixated on their sleep scores. There's even a medical term for an unhealthy obsession with ideal sleep: orthosomnia.