Smart wearable health devices have become an important part of modern healthcare systems. They make it possible to keep track of things like heart rate, blood oxygen saturation (SpO2), physical activity, and sleep patterns all the time and in real time. Even though these devices are used by many people, it is still very important to be concerned about their accuracy, reliability, and long-term performance. This review paper offers a thorough comparative analysis of top smart wearable health devices by assessing their sensing technologies, data accuracy, reliability across diverse conditions, and overall performance metrics.A systematic literature review methodology is utilised, integrating studies from prominent scientific databases and recent developments in wearable technology. The analysis shows that different types of devices have different levels of measurement accuracy. It also looks at the factors that affect reliability, such as sensor quality and environmental conditions. Finally, it looks at how the wearable health industry has changed because of improvements in the Internet of Things (IoT), artificial intelligence, and small biosensors. Additionally, this paper addresses contemporary challenges such as data privacy issues, regulatory constraints, and device standardisation challenges. The results show what is good and bad about current devices and suggest ways to make wearable health technologies more accurate, reliable, and integrated into both clinical and personal healthcare settings.

Index Terms—Smart Wearable Devices, Health Monitoring, Accuracy, Reliability, Internet of Things (IoT), Sensor Technology, Digital Healthcare