Smartwatches and fitness trackers are often not accurate, as they don't directly measure these key health metrics directly. Calorie tracking is a popular feature of smartwatches, but its accuracy can vary significantly. They can under- or overestimate energy expenditure (often expressed as calories burned) by over 20%. Errors can also vary between activities such as strength training, cycling, and high-intensity interval training. Smartwatches can also under-count steps by about 10% under normal exercise conditions, but this is still useful for tracking general activity levels. However, step counts can be less accurate due to the use of arm movement and tight adherence to the watch's heart rate. Most smartwatch users use heart rate variability and sleep quality to create a "recovery" score, which may not accurately reflect their recovery. Smartphones can also estimate sleep using movement and heart rate, but are less accurate at identifying sleep stages.