Optimize your workout routine with a Daily Readiness Score that reveals if you’re ready to exercise or should focus on recovery (Requires Fitbit Premium membership). Compatibility-Apple iOS 15 or higher, Android OS 9 or higher .Operating Temperature : -10°C to + 45°C.Radio transceiver : Bluetooth. Get a daily Stress Management Score showing your body’s response to stress and take steps to improve your levels with an on-wrist EDA sensor mindfulness session. Band Size:Sm: Fits wrist 5.1"- 6.7" . Lrg: Fits wrist 6.7" - 8.3" in circumference Track your heart health with high & low heart rate notifications and a compatible ECG app (The Fitbit ECG app is available in select countries. Not intended for use by people under 22 years old.) With the Health Metrics dashboard, track SpO2, heart rate variability, skin temperature variation and more (Not intended to diagnose or treat any medical condition and should not be relied on for any medical purposes.) See your real-time pace & distance without your phone using built-in GPS during outdoor activity, then see a map of your workout route in the Fitbit app Your stats come to life on a new color touchscreen that’s two times brighter than Charge 4 in daylight, all with up to 7-day battery (varies with use and other factors) Track calorie burn and optimize workouts with 24/7 heart rate tracking and Active Zone Minutes, which guide you toward your desired intensity level Get a better understanding of your sleep quality with a daily Sleep Score and graphs of your time in light, deep and REM sleep—then see how you can improve your sleep and wake up feeling energized Includes a 6-month Premium membership complete with personalized insights, advanced analytics, guided programs and more (New & returning Premium users only. Must activate trial within 60-days of device activation. Content and features may change)