You need to watch the pricing of your various television spots, as well as, when they air.  For station XYZ, late night runs from 11pm to 2am and costs $1200, while overnights run from 2am to 5am and cost $300.  If your spot airs at 1:53 am, and you are charged $1200, it is highly unlikely that your spot will be profitable as the number of viewers towards the end of late night is substantially smaller than the number of viewers during the beginning of late night.  A good media buyer will be watching this for you, but, as with everything, its important to watch the details.