The OP probably took MH4 which would be a complete day flight, departing KUL at about 10 am and arriving the same day at 4 pm in LHR. Being so, the window shades generally remain open and hence it is difficult to sleep regardless whether the cabin lights are dimmed.

I took MH4 once, and having connected from a 4 am flight from CGK, it was a dreadful experience. Add to that the state of MH's A380 cabins which I'd argue are not maintained too nicely.