n01 writes: Researchers of the University of Washington are testing the prototype of their ApneaApp to diagnose sleep apnea, a health problem that can become life-threatening. To monitor a person's sleep, the app transforms the user's smartphone into an active sonar system that tracks tiny changes in a person's movements. The phone's speaker sends out inaudible sound waves, which bounce off a sleeping person's body and are picked back up by the phone's microphone. "It's similar to the way bats navigate," said Rajalakshmi Nandakumar, lead author and a doctoral candidate in the UW's department of computer science and engineering. "They send out sound signals that hit a target, and when those signals bounce back they know something is there." In technical terms, the app continuously analyzes changes in the acoustic room-transfer-function (sampled at ultrasonic frequencies) to detect motion. This is very similar to what the iPhone app Sleep Cycle Sonalarm Clock does, except that the UW researchers have improved the sensitivity of the method so it can precisely track the person's breathing movements which allows it to not only detect different sleep phases but also sleep apnea events. The advantage in both use cases is that the sleep monitoring is contact-less (there's nothing in the user's bed that could disturb their sleep) and doesn't require any additional hardware besides the user's smart phone.