Does the Band SDK expose information on whether or not the wearer is currently awake or asleep?
I know this information is provided to the user via the Microsoft Health app in their daily graph, but as a third-party developer can I get this information?
No, the Band SDK does not expose this information.
You can however get the info from the Microsoft Health Cloud API, check out the documentation: https://developer.microsoftband.com/cloudAPI/Explorer#!/Activities/Activities_Get
The sleep data will not be provided in real-time by the Cloud API - as I read the documentation it will be delayed with minimum 1 hour.