You can use a band that’s designed for Apple Watch (1st generation) or Apple Watch Series 1, 2, and 3 with Apple Watch Series 4, Apple Watch Series 5, Apple Watch SE, Apple Watch Series 6, and Apple Watch Series 7, as long as the sizes are compatible. This watch is a combination solar-powered watch that contains a solar cell in its face that drives the watch hands, race timer, chronograph, alarm, and numerous other functions by converting light energy into electrical energy. A Manual Wind Watch is a mechanical watch that does not require a battery, but can only be wound by hand. To wind it, turn the crown clock-wise while it is in its normal operating position until you feel resistance. Clean the back of the watch from time to time with a few drops of water and wipe it dry with a cloth to remove any grease or oils. If the message "do not move" is displayed, try to slow down your body movement in order to take a more accurate heart rate measurement. The heart rate measurement may not work correctly if you have an irregular Your watch analyzes your wrist heart rate readings while you are sleeping to determine your heart rate variability (HRV). Training, physical activity, sleep, nutrition, and healthy habits all impact your heart rate variability. Mechanical watch characteristics (Hand-wound, auto-wound) This is a mechanical watch powered by a spring. To use it from a stopped state, turn the crown 20 turns by hand to wind up the spring before starting the watch. E820/E821 Abbreviated instruction This watch is solar-powered. Expose the dial to light and use it sufficiently charged. To see details of charging time, specifications and operations, refer to the instruction manual: E820/E821 instruction manual