The 'sugar crash' after dessert isn't quite what it seems
It's widely believed that a big hit of sugar triggers an equally big insulin spike that 'overshoots' and drives blood glucose down too far (reactive hypoglycemia), causing fatigue, irritability, and cravings for even more sugar — the so-called 'sugar crash.' In healthy people without carbohydrate metabolism issues, controlled studies that actually measure blood glucose after sugar intake find this kind of pronounced reactive hypoglycemia far less often than the popular belief suggests.
The fatigue people feel after something sweet is more often explained by other factors: the natural post-lunch energy dip tied to circadian rhythm, the psychological effect of expecting a 'crash' after reading about the phenomenon, or simply the total amount of food eaten diverting blood flow to digestion, regardless of its specific sugar content.
This doesn't mean excess sugar in the diet is fine — high intake of added sugar is tied to other real problems for health and weight control. But specifically as a sharp physiological mechanism in healthy people, the 'sugar crash' is largely an overstated and not-quite-accurately-described phenomenon.