I agree with KHW. I added this explanation on another question I just answered about "Why did'nt Anaj Ventress use Force lighting". I was talking about how the continued use of the dark side makes you ugly in apperance as Palpatines true form was. His human looking kindly old man appearance in the prequels is realy a mask of dark side energy. Here is what I wrote below;
"As others mentioned too, there is the draining of that persons appearance, soul and life force or chi when using the dark side at all, not to mention when throwing force lightning. In other books, it was taught that any use of the dark side eventually casuses the splotching and bruising seen in the Emperors appearance. But in ROTS we saw that in palpatines case, it was because Mace was using Vapaad to repulse the dark side lightning and, along with his lightsaber, send it back towards Palpatine.
I personally thought that Palpatines entire human like appearance was a force mask, and that the lightning simply revealed his true visage. I'm not sure if I read that in ROTS or in another book, but I definitely remember that being a possible explanantion.
In other words, his current decrepit appearance IS his normal appearance through the decades of use of the dark side, and his human looking one was simply an illusion. Luke was also able to totally mask his appearance where others could not tell at all who he was. Only other high level force users were able to tell that Luke was using the force to hide his true form."