What time is the shuttle bus available on Sunday morning

the BWI web site says shuttles run every 5 to 10 minutes except for early morning hours 1am to 5am when it's approximately every 25 minutes. this is not a strictly scheduled service, so allow yourself some flexibility in time during the early morning hours

It also depends on which shuttle you are using

Depends if it's a holiday or not. They are usually running pretty early.