Put your PC to sleep when you go to sleep as well. PC won't access HDD when it is put into sleep and there's no HDD LED flicker.
Or you can always cover the power button and it's surrounding LED light with paper/magazine so that you won't see it.
For 3rd solution, pull the HDD LED connector plug out of your MoBo, so that the HDD LED won't lit up at all.
It's the 5th wire from the left in the following image: