Lab Timings?

I have a question:

Do you require your students to be in the lab at certain times? If yes, what is your policy?

For me my advisor was very open to timings. He never required us to be in the lab at any of the hours.

However, we were always there.

My students however, decide not to show up. They show up whenever they want. This bother me. I am not sure why. I think I feel they are not working when they are not in the lab.

However, I am in the lab all the time since my office is right in the lab.

Shouldn't the students putting in more efforts to get ahead in the research game and may be impress their professor?