We have investigated self-assembled InAs quantum dot superlattices using photoluminescence and transmission electron microscopy. We report results regarding the influence of the dot vertical separation on the optical properties of such structures. The photoluminescence peak shifts toward higher energies and its intensity drops by one order of magnitude when the distance between two consecutive quantum dot layers is reduced below 70 Angstrom. Our transmission electron microscopy images suggest that such unexpected photoluminescence features are related to the formation of structural defects induced by the large amount of strain relieved at small dot separations.