I suppose it would affect the dark period and maybe slow the growth of your plant/s. The idea is to simulate day and night with your lights and to the best of my knowledge you don't see flickering lights in the night sky so the tv might affect your plant, but on the other hand you do get moon light on outdoor plants sooooooooo your best bet is to just hang up a bed sheet or something similar to stop the tv light reaching your plants.
Are your plants growing in your room or a closet?