I have noticed this before although i switched to using the 32 bit version of minecraft as it didnt have the unfinished rendering engine which is the renderdragon engine. Since both 64 bit and 32 bit have renderdragon now I dont have any other way to go back to the older rendering engine. But it has also come to my attention that there is a bug in using sky overlay texture packs. At night time it is supposed to make the sky darker as it is night of course but with this rendering engine it doesnt do that. It simply stays the same making the sky brighter that what it is supposed to.