Apple Watch Series 8

Two sensors and a car crash. The Series 8 added skin temperature sensing using two sensors, one against the wrist and one under the display, sampling every five seconds overnight to detect changes of as little as 0.1 degrees Celsius. Apple used it for retrospective ovulation estimates and cycle tracking, which is genuinely useful health data that a watch is well placed to gather while you sleep. Crash Detection arrived at the same time, using a high-g accelerometer and gyroscope to recognize a severe car accident and call emergency services. Everything else came from the Series 7. It is a small update carrying two features that matter enormously to the people they apply to.