EDA Scan app detects electrodermal activity which may indicate your body's response to stress and a built-in skin temperature sensor logs yours each night so you can see when it varies.Band Size : Small : Fits wrist 140mm - 180mm in circumference. Large : Fits wrist 180mm - 220mm in circumference Assess your heart for atrial fibrillation a heart rhythm irregularity- and easily share results with your doctor (The Fitbit ECG app is only available in select countries. Not intended for use by people under 22 years old.Operating temperature: 14 to 113F An on-wrist skin temperature sensor tracks yours each night so you can see how it varies. You can also see your nightly blood oxygen levels at a glance with our collection of clock faces.Maximum operating altitude: 28,000ft High low heart rate notifications alert you if yours seems above or below your average Battery lasts 6 plus days plus, fast charging gives you a full days charge in just 12 minutes (Varies with use and other factors; up to 12 hours with continuous GPS) Use built-in GPS during runs, hikes, rides and more to see pace distance without your phone and use the built-in mic and speaker to take Bluetooth calls hands-free when your phone is nearby (Requires more frequent charging) Use Google Assistant or -- Alexa Built-in to get quick news, set bedtime reminders and alarms, control your smart home devices and more just by speaking to your watch