An activity tracker of some kind is an absolutely essential tool for any age reversalist. You can’t just guess at how many calories in a day you are burning. Or how far you ran or cycled, and at what speed or pace. It’s all so important. Remember guiding principle number 4 which states, “Everything matters; what one eats, when one eats it, type of exercise, duration of exercise, intensity of exercise, state of mind, what one thinks, and how one thinks. Everything! The seemingly smallest details matter. Changing them can have profound effects, even if we cannot understand why. Consequently, I will document as much as is reasonably possible.” Having the data an activity tracker provides has enabled me to quickly make adjustments. Of equal importance is its ability to help discover correlations. Without my activity tracker, I would never have known that consuming alcohol raises my resting heart rate and heart rate variability for a day or two.