One of the main ideas from Apple Watch is to be a companion device for your iPhone. Apple Watch and iPhone are connected via WiFi and Bluetooth. They will send data or sync app each other. The data sent by iPhone in the form of notification of certain application or data that comes from the sensor of an iPhone for fitness needs and the Apple Watch downloads the sensor data gyroscope, accelerometer or barometer from your iPhone. So does with Apple Watch, it sends certain data to iPhone. If you have read an email on Apple Watch, your mail app on your iPhone will mark that email as read automatically.
This two-way communication can be different for every user. The data size and sync repetition that have to be done is a relative thing. So, it’s not weird if some people experience that their iPhone’s battery life decreases while they are using Apple Watch which is connected to the iPhone, while others didn’t experience significant change. If we see the scheme of Apple Watch and iPhone “activities”, the iPhone’s battery life will decrease, because of the additional work which needs more power. If both “activities” are low, the user won’t notice a significant battery life decrease.
One of the important things is the software update for iOS, and OS, because there must be a communication bridge that can save the battery life. We are still waiting for a new Bluetooth technology with the low power for communication device with a wide bandwidth. It seems this will happen soon, which will be great for sure!