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.

Early (or late) meetings are generally unreasonable. Some people naturally wake up early, while others stay up late and are more productive in the evening. If you force people to attend meetings that are too early or too late for them, they will be physically present but may not contribute that much due to being too tired.

What constitutes an early or late meeting depends on the society and culture. At the institute I currently work at, people are generally expected to be around from 10:00 to 16:00, and regular meetings outside those hours would be unreasonable.

It is not unreasonable, but it might annoy your graduate students. I myself wouldn't like it because it would mean that I have to get up an hour earlier than my girlfriend.

Maybe one should set the question the other way round: What is your disadvantage if you meet at nine? Is it important for you to meet your students first thing in the morning?

Comparing your need and your students needs, you should find a sensible compromise.