Is 8:00 AM an unreasonable time to meet with my graduate students and post-doc?
It depends entirely on whether you have consulted the other meeting participants. There is nothing inherently wrong about an 8 a.m. meeting if it is convenient for everyone involved.

If you have set the time without consulting the other participants, there are two serious problems. 

You may be giving one or more of them a practical problem. For example, consider a parent whose child's school starts at 8 or later. You may be disrupting carpool arrangements. It may conflict with a class one of the students wants to attend.

The other problem is more subtle, and applies even if none of the participants actually has a problem with the meeting time. Setting the time for a small meeting without consulting the other participants and considering their views is very disrespectful. It tells them that you consider your schedule, your convenience, to be so much more important than theirs that you don't need to consider them at all. That sort of message is not good for a working relationship.