'Sleep Cycle'. Orange icon with a clock on it. I was always quite skeptical of it and didn't download it until recently. It's been one of the top apps for a few years and has good reviews. I've found it to be quite accurate. It recommends that you keep your iPhone connected to a charger and under your sheet as you sleep but I've found just placing the device next to my pillow works fine and only uses about 25-30% battery life per night.