  Spectral energy distribution and its variability are basic tools for
understanding the physical processes operating in active galactic nuclei (AGN).
In this paper we report the results of a one-year near infra red (NIR) and
optical monitoring of a sample of 22 AGN known to be gamma-ray emitters, aimed
at discovering correlations between optical and gamma-ray emission. We observed
our objects with the Rapid Eye Mount (REM) telescope in J, H, K, and R bands
nearly twice every month during their visibility window and derived light
curves and spectral indexes. We also analyzed the gamma-ray data from the Fermi
gamma-ray Space Telescope, making weekly averages. Six sources were never
detected during our monitoring, proving to be fainter than their historical Two
micron all sky survey (2MASS) level. All of the sixteen detected sources showed
marked flux density variability, while the spectral indexes remained unchanged
within our sensitivity limits. Steeper sources showed, on average, a larger
variability. From the NIR light curves we also computed a variability speed
index for each detected source. Only one source (PKS 0208-512) underwent an NIR
flare during our monitoring. Half of the sources showed a regular flux density
trend on a one-year time scale, but do not show any other peculiar
characteristic. The broadband spectral index alpha_ro appears to be a good
proxy of the NIR spectral index only for BL Lac objects. No clear correlation
between NIR and gamma-ray data is evident in our data, save for PKS 0537-441,
PKS 0521-360, PKS 2155-304, and PKS 1424-418. The gamma-ray/NIR flux ratio
showed a large spread, QSO being generally gamma-louder than BL Lac, with a
marked correlation with the estimated peak frequency nu_peak of the synchrotron
emission.
