Exactly. And there will always be this kind of mismatch between what the human wants and what the device can anticipate as long as the human is smarter than the device. Even among humans it takes years of learning social norms before we are able to anticipate each other with any degree of accuracy.
People always get upset at these "smart" devices, and decry their stupidity, and yet they wouldn't really do that much better of a job were they in the device's shoes, so to speak.
Not all people have the same sleeping pattern. While 9PM may be cool enough time to remind you to recharge the battery, it will be a disaster for other people. I think either of following will be a great feature to add: 1) auto-recognizing sleeping pattern of watch wearer (may be hard to implement); 2) just to add an option in system setting your usual sleep time.