Worth remembering, especially since I don't like Cadbury eggs. I think the fake egg with the yellow center is very charming, but they taste much too sweet for me.
Just commenting to say that I have also noticed the effect you've mentioned of too much sugar negatively impacting motivation. I recently gave up all food/drinks that have sugar/sweetener in the first three ingredients and it's made a noticeable difference.
Odd complementary anecdote: I just started the ketogenic diet again and am noticing that my motivation and cognition are getting better, despite being in a depressed state.
Yes there is research that supports that high glycemic index (GI) food such as sweets and white bread deteriorate willpower. The main mechanism that is made responsible for this is the drop in blood sugar a few hours after consumption. This post has some points on it: http://lesswrong.com/lw/cmc/book_summary_willpower_by_baumeister_tierney/
More info on this can be found in the book 'The Willpower Instinct' by Kelly Mcgonigal. She says the research most supports plant-based diets and low-GI diets. Evidence for the former is better.
Furthermore I think I've heard a few things about 'hyper rewards' such as pornography, drugs and sweet food generally reducing motivation. This makes sense since these rewards were not available in ancestral times. The brain responds to strong stimuli by downregulating the neural pathways required for motivation. This should be easy to google.
I don't think what's affecting me is the immediate blood sugar drop-- the problem takes a few days to clear up, and when I'm in the middle of being demotivated, eating (whether high or low glycemic food) doesn't make a big difference.
I've personally noticed something somewhat opposite -- small amounts of sugar water (like 8g sugar per cup of water) seem to keep me motivated. But I don't eat much sugar otherwise.
I reread Are Wireheads Happy?[1], which is about the difference between liking and wanting.
I've noticed that too much simple carbs (not terribly much-- three twenty ounce Cokes on consecutive days will do it) severely damages my desire to do much of anything, and the effects take approximately two low-simple carbs days to clear out.
This is obviously something physiological, even though it looks like an emotional problem. Failing to remember clue to avoid simple carbs (a dessert once a week might not be a problem) might be an emotional issue. Too much sugar used to lead to an internal voice saying "I don't care". This time, it didn't, but if I thought of something I might do, there was a clear feeling of "no reward there" and a sense that it was too much effort. I was capable of enjoying things, but not of anticipating that I would like them.
I'm wondering if anything is known about simple carbs, motivation, and/or serotonin/dopamine.
[1]Recommended here.