The central question that arises when discussing the performance of wearable devices—especially those used for monitoring sleep and activity—is how “well” they actually work. This question is fundamental and frequently posed, as it directly relates to the reliability, accuracy, and utility of these devices in various practical applications. To address this, we must first understand how algorithms and scoring strategies were developed and employed in early devices. The design and evolution of these algorithms underpin the functionality of more advanced, modern devices, and algorithms used today are built on the foundations of these earlier methods. Examining their historical development provides helpful insights into how technology has advanced and how it continues to evolve.