MATLAB: Standard deviation for a field inside a field

Hey guys,

I have a 1×400 struct, wherein this struct is a field called 'BO', this field called 'BO' contains a struct, which in themselves is a field called 'circles' that ranges from 1:20 rows (they're all different values). How can I access the standard deviation of each of those circle fields? Do I apply a cell conversion or is it possible to leave the data like this?

Thanks!