  We study the variance of the number of zeroes of a stationary Gaussian
process on a long interval. We give a simple asymptotic description under mild
mixing conditions. This allows us to characterise minimal and maximal growth.
We show that a small (symmetrised) atom in the spectral measure at a special
frequency does not affect the asymptotic growth of the variance, while an atom
at any other frequency results in maximal growth. Our results allow us to
analyse a large number of interesting examples.
