I've come to a different conclusion. I used to think it was a "fuel" problem until I realised that if I moved onto some other non work related task that I was interested in, which could include programming, all of a sudden fuel wasn't an issue. For me it came down to motivation and interest with "mental fuel" rarely being the deciding factor. I manage to conjure up fuel when I'm sufficiently motivated by a problem. Obviously this might not be the case for everyone and I have hunch it is related to ADD behaviour I exhibit (self diagnosed).

Tricking my brain into investing into the outcome of work I am not interested in takes up most of my time now trying to get over procrastination. And of course I have a deadline today and I'm writing this message on HN... but like any good developer that needs some pressure to work I've got the plates all spinning for now.