Not merely how well you sleep and how it can vary according to factors you can record at bedtime. Also wether you snore or talk in your sleep, and what you say. The graphs plotting "sleep quality" versus "weight" are clinically significant only if you have weight-induced sleep problems, likely if your BMI is 35 or more. For the comparison to heart rate, an iWatch is best, since it monitors your wrist pulsations continuously and allows the app to compare it to the movement and audio data it monitors while under your Buy Derila Pillow. This will expose a side of you that you knew nothing about, fascinating in itself, and may warn you of upcoming or ongoing health problems. A truly useful app. It is one of the first things I look at EVERY morning! It also provides detailed info on how your behaved during the night, indispensable for persons with rate or thutm problems.