A two-state wearable device may include a wearable band (e.g., a watch band) and a display module (e.g., a watch face). In a “detached” state, the wearable band and display module act as separate devices, and the display module can be charging its battery while the wearable band gathers sensor data (e.g., sleep tracking data) and provides a vibrating/audio alarm clock function. In an “attached” state, the wearable band and display module act together as a single device, meaning that the data collected by the wearable band during the “detached” state is sent to and stored at the display module, and that sleep settings input at a user interface of the display module impact the wearable band's alarm function. The display module may also use its newly-recharged battery to recharge the wearable band's battery so that it can be used the next night.