Sleep scientist Patrick Fuller notes that sleep apps can’t monitor what’s going on inside the brain the way a sleep study can, reports Tech Insider.

Some sleep apps claim to measure and analyze your sleep cycles by using the phone’s accelerometer to detect your movements during the night — for example, if you’re moving around you’re in a lighter sleep stage, and if you’re as still as a log you’re in a deep stage. But according to Fuller, it’s near impossible to diagnose the quality of your sleep based on movement alone.