Is 8:00 AM an unreasonable time to meet with my graduate students and post-doc? I am a recently graduated PhD, but for the past few years I have been on an 8:00-16:30 schedule with a lunch break at noon.  I arrive between 7:45-8:00 and get started immediately.

Now I am on as a continuing research scientist and have been assigned to supervise two graduate students and another post-doc that's new to the field.  It's important for our schedules to overlap, so I've instructed them to meet me at 8:00 to get started.

To clarify the frequency, this type of "meeting" would be once a week for the next 13 weeks. I use quotes because it's really just the time we agree to get in the lab and start work together, which is still a meeting but not the one we often think of when we see the word.

Someone I am close to, who has both graduate school and office (industry) experience, tells me this is unreasonable.  According to her, even in formal business environments, the first [half-]hour is reserved for checking your email, getting settled, going through voicemails, etc.

As I like to tell people, I was a student for 25 consecutive years, so I know the ropes regarding when academics typically wake up and get started.  I'm not asking if starting work at 8:00 AM is typical, but rather if it is unreasonable and why I should or shouldn't change my policy.
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.