  We discuss the variation of gauge couplings in time in the framework of
orbifold constructions, due to a change of the extra compact dimension's size.
Models with gauge coupling unification allow to estimate the variation of the
strong coupling constant $\al_3$ and to relate it to a variation of
$\alpha_{em}$. The extra-dimensional construction turns out to be crucial for
the model to be compatible with data. Within the presented 5D scenarios, the
tower of KK states significantly affects gauge coupling running, leads to low
scale unification, and provides a suppression of the $\al_3(M_Z)$ variation.
