The Fermi energy and the Zeeman splitting of composite fermions are measured from the temperature dependence of the electron spin polarization at $\nu=1/2$. We demonstrate that the Zeeman splitting of composite fermions is enhanced by a factor of $2.5$ due to the interaction between CFs. The latter is very sensitive on the finite width of the $\mathrm{2D}$ channel. The spin polarization at $\nu=1/3$ and $\nu=2/3$ displays an activated behavior and the derived spin-wave gaps are compared with simultaneously measured transport values.