Maybe I should know the answer to this by now - but.. Why does high blood sugar make people with diabetes sleepy? I know that high blood sugar is a sign of insulin deficiency or resistance... but... I'm interested in the effect going the other way. I mean I know that... insulin deficiency/resistance = poor utilization of blood glucose = tiredness, I get that. But what about if a diabetic requires the same amount of energy as they would normally, has the same insulin levels as they would normally, but just eat more cake? Why is it that raising the blood sugar level per se increases tiredness, irrespective of how much energy that person needs? Any thoughts gratefully received!