Ok, well, temperatures have been low over the past couple of weeks, and if apartments above and below yours are unheated for a period of time it is most likely that you will notice a significant drop in temperature as more heat is lost from your apartment to those (temporarily) unheated spaces. Apart from sealing any gaps in the construction there is very little you can do, but make sure that your heating system (maybe this is electric only?) is properly serviced and running efficiently. monitor the situation and see if matters improve once those apartments are occupied once more. The usual advice regarding fitting heavy, lined curtains etc is as valid as ever of course.