Number of calls of hook_node_access I use hook_node_access() to redirect users, if they don't have the right attributes to see a specific node.

While programming I used dpm() to check some variables and was wondering, that dpm() is called 9 times although I just opened a single node. 

Well, my question is:
Is it normal, that hook_node_access is called several times for one node, so I should use drupal_static() to avoid duplicate calculations or do I have some disadvantageous loops in my code that I should detect and remove (in hook_node_access are no loops, I mean somewhere else)?

I would welcome every advise!

Thanks, Daniel
Next time I should look into the docs first...

According to node_access(), it doesn't consider the returned value NODE_ACCESS_IGNORE, which I return, if the user doesn't violate my permissions, so that other modules still can deny access, if their permissions might be violated.

So the answer of my question is: I should use drupal_static() to avoid duplicate calculations.