I did not have an Rny but your question made me curious so I went to Google and found this very insightful response to another person who had a similar question.  I hope it helps.  I have pasted the text from website in case the link does not work.
http://askville.amazon.com/sugar-make-sleepy/AnswerViewer.do?requestId=8590207

Why does sugar make me so sleepy?

It seems like any time I eat anything with very much sugar in it, it makes me very sleepy almost instantly. I react almost like I had taken a sleeping pill. Almost immediately after eating sugar, if I'm not stimulated by visiting with someone, I fall asleep almost before I know it. My doctor tells me I'm not in any danger of diabetes.

 More search results: sugar makes me sleepy Asked by catnanny 47 months ago       Similar questions: sugar make sleepy Health > Fitness & Diet

Doctor, shmockter.
I am hypoglycemic. I have had a 5 hour glucose tolerance test. Doctors who practice traditional as well as alternate medicine tell you that it takes a 5 hour test to get an accurate reading of the glucose/insulin reaction in the body.
Fasting, you arrive at the doctor's office. They give you some glucose to drink and then every half hour they test your blood and you also hand over samples of your urine. This goes on for five hours. They chart your sugar levels, and find out when your body produces insulin to counter the sugar and how much or how effective it is. If you produce too much insulin too quickly, you will get that low feeling. Rarely, some people, really bad, collapse in the office. Most doctors do not want patients hanging around in the lobby taking up all the seats for 5 hours. 
If you are bad enough, find a holistic or complementary physician who will give you this test, or, you could just do as I now do, which follows. 
People who have a problem metabolizing sugar are supposed to, classically, have an immediate high from the sugar followed by exhaustion. The tiredness comes because their bodies are producing too much insulin to handle the sugar, thereby depleting the body of sugar and causing tiredness. 
The reason these people are called pre-diabetic is that the glands that over react in producing this abundance of insulin will eventually just give up, causing you to not handle the sugar in your body and become diabetic. Diabetes is nothing to fool with. If I am reacting badly to sugar, I would do something about it. If you are diagnosed with diabetes seven years down the road, you would wish you would have handled it better right now. If you find you have diabetes later, your doctor would just tell you that you didn't show any signs of it right now. Some diabetics are hard to control. They can lose sight, legs. They are much more prone to heart problems. Everything that goes in their mouths must be evalluated. They are told to lose weight and given insulin or drugs that cause them to gain weight. 
Better to address something now, being (in my opinion) lucky enough to have a forewarning. 
My advice, learn which sugars are the worst for you: soda, candy, cookies, cakes - then fruit juices - then fruit - then an entire list of things on the 'glycemic index' that any diabetic should almost have memorized, but you may not have to.
Study how many diabetics there are in the US. That does not happen overnight, usually. It happens because we are too fat and because there is sugar added to so much that we ingest. 
One thing that ABSOLUTELY helps me, and may help you - I know there are other brands, but Swanson's Cinnamon in capsules really eliminates those highs and lows for me. My husband takes it daily. I just take one capsule before eating something sweet. 
There are different sorts of cinnamon, and I don't know if they all work the same, but I actually found out about it in a science class from the instructor. It has evidently been tested. 
Ideally, I would just avoid sugar as much as possible, but if you do take it, try the cinnamon. 
If you are interested in learning more, I would start with the South Beach Diet book that explains sugar metabolism to you in an easy, almost fun, way. I would not buy the diet book, because I am not one to do the recipe thing, but it is probably a good follow up if you like that sort of thing. Your library may have both of these books. You could also try an Atkins book. His diet is considered extreme to some, but his advice is basically good. Just be careful of all the fats he allows you. South Beach is a modification of Atkins and most people think it is healthy. 
REad labels. Do you know that Americans are fatter and have more diabetics than Europeans? Do you know how many of our foods contain 'high fructose corn syrup'? Do you know that 'high fructose corn syrup' is illegal to use in foods in most of Europe? 
Most things with 'ose' as a suffix are sugars. Check labels.
It is nearly impossible to get a high fibre cereal with really low sugar. 
Most are wheat based. 
People with food sensitivities - well - the three most reactive foods are milk products, wheat, and peanuts. You can have a sensitivity to one of these, unknowingly keep eating it, and it will interfere with your nutrient digestion of other foods. 
Hope this helps a bit.  

Well...thanks for that pearl of wisdom!

 Now I'm not an expert on this, and if you put it in the search at the top, you'll get a lot of posts with better info. but;  it sounds like something I've heard of here called reactive hypoglycemia. I am not familiar enough with it to give advise, but I suggest you put it in the search bar, and I know those symptoms sound like they correlate to that. (Try a protein bar instead of the candy bar, 'cause they come in those "chocolate" flavors too, and might give you the same satisfaction without the reaction of sleepiness)