Weighing in from the medical angle-both Bale and Ledger came down with severe viral pneumonia during the filming of Batman. Both started having issues with insomnia after coming down with this viral illness-prehaps both are suffering from a form of post-viral syndrome (a milder form of Guillain-Barre disease). Working in England is a great way to pick up odd and obscure viruses, because of immigration, proximity to the former USSR (where many diseases flourish unchecked because of their lack of health care system). And picking up a nasty virus while doing something incredibly physically and psychologically challenging such as a movie like Batman, is a great way to make sure you are ripe for all the complications.
If you google post viral syndrome you will see that it tends to be dumped in the same wastebasket as fibro. But there are many scientific studies that show this to be a very real illness.
Of course there is also the issue of "survivor's guilt" which is strictly a psych problem....here the man is being faced with his dead friend's image every day, that can't be good for him either.
