What's your threshold for too much sugar PL? I suppose if you made them yourself it would be a little different but I would imagine most store bought flapjacks would be generously sugared.
Sorry - only just noticed this. I spotted from a separate post that you're vegetarian, so you probably already know a lot about nutrition. What follows isn't intended to be patronising! But if you're not familiar with this it may help, so here goes.
Basic carbohydrate chemistry; think of carbohydrates as a number of carbon rings bonded together. The simplest consist of a single carbon ring and are called monosaccharides, such as glucose. Two carbon rings bonded together are called disaccharides (such as sucrose - common table sugar). Starch is an example of a polysaccharide, with about 1000 carbon rings bonded into a far bigger molecule.
Obviously, the time taken for your gut to digest carbohygrates depends on the size of the molecule; starches take far longer for enzymes to pull to bits than small molecular size sugars. Ultimately everything is broken down into glucose, which is very soluble and easily absorbed into your bloodstream.
This is where the problem lies; if you east a lot of table sugar (which is what KMC is essentially made of) its two carbon rings are quickly split into glucose molecules, which are then quickly absorbed and your blood glucose level skyrockets. This is VERY bad for your body in a great many ways - if you've ever seen a hyperglycaemic diabetic you'll know exactly what I mean.
In healthy people a hormone called insulin is rapidly secreted by the pancreas in response to the fast rising blood glucose concentration. Insulin has the effect of removing glucose from the blood (and storing it as a polysaccharide galled glycogen, e.g. in the liver, where it's stored for later release when needed).
Bear with me; we're almost there . . . . (!)
A very fast blood glucose spike can result in an oversecretion of insulin, which then lowers blood glucose level BELOW what it should be. So eating a load of simple sugars can actually result in a LOWER blood sugar level than beforehand! The result is you get a brief lift but then you quickly feel like you've hardly any energy.
Now to your question; counterintuitively, gobbling down a load of KMC can result in you feeling tired. Munching a flapjack (which consists mainly of starchy oats, which take time to digest, slowing down the release of glucose) will smooth out the energy delivery and not cause you to oversecrete insulin. This keeps your blood sugar level nearer to what it should be to provide the optimum amount of energy for muscle cell respiration. The sugar in the flapjack gives you a quick lift but it's lower proportion in the recipe is less likely to cause a massive sugar spike which would lead to the problem described above.
The one caveat I would give is nibbling tiny
amounts of KMC regularly (rather than gluttoning down whole bars when you suddenly feel hungy) is probably OK.
This is why I'd recommend foods with a higher starch content and lower sugar content as caving snacks.
Does that help?