The latest Apple Watch includes a new power-saving mode that is not user-accessible.

Apple is limiting who can use Adaptive Power mode on Apple Watch, maybe for a good reason.
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Adaptive Power mode can be used on your child's Apple Watch
The new power-saving feature is included in watchOS 26 and is automatically enabled on all compatible Apple Watches configured for children. Adaptive Power mode is also automatically activated on previously configured Apple Watches for kids that receive the watchOS 26 update.
Apple Watch For Your Kids, previously known as Family Setup, allows parents to set up an Apple smartwatch for their children. The children’s setup is only possible on cellular-enabled models and is intended for children who do not have their own iPhone.
The Watch function, as described in a new Apple support document, works similarly to its iPhone counterpart. The goal is to extend battery life by optimizing the watch's performance. Apple cautions that this may cause Siri to take longer to fulfill requests and some animations and scrolling to appear less fluid.
It is not a required feature
Although enabled by default, Adaptive Power on a child's Apple Watch can be disabled. This can be accomplished in the Battery section of the Settings app.
Adults are unable to use the feature on their own smartwatches. Adaptive Power is only available on devices configured with Apple Watch For Your Kids.
Adaptive Power is useful on children's watches
Apple's restrictions on Adaptive Power on Apple Watch may seem strange at first, but they are logical. For most adults, the Apple Watch is an iPhone accessory, so a simpler battery-saving mode should suffice for everyday use. However, for children who rely on a cellular Apple Watch as their primary mode of communication, having it enabled could be critical. It would still be beneficial for Apple to expand the feature to more devices, but I doubt I would miss it as much as a child would.