[BaO]

i've also heard that sugar makes us sleepy?
does it make us fat?
is "fat" the only one that makes us fat?

[mike]

Do you mean sucrose?

[AWK]

Sugar has about twice times lower energetic value than fat. Both sugar and fats can be converted in human body into human fats (both through degradation to acetic acid, and then building fatty acids).
I do not believe in sleepy properties of any sugar, except for diabetic peoples (as far as I know - only glucose can cause diabetic coma))

[Yggdrasil]

Eating lots of carbohydrates can induce sleepiness through a temporary hypoglycemic state (low blood sugar levels).  When a person eats lots of carbohydrates (not limited to sucrose, but starchy foods as well) the breakdown of these foods into gluclose will caulse a person's blood sugar levels to rise (hyperglycemic state).  The pancreas will respond by producing insulin to promote the uptake of insulin.  However, the insulin will do its job too well, resulting in a temporary hypoglycemic state which causes the sleepiness known as"food coma."

I've also heard, though that food coma may also be caused by the body redirecting blood flow to the stomach in order to digest the large amounts of carbohydrates eaten.  In this case, the reduced blood flow to the brain would be the cause of sleepiness.