Steady glucose levels help support steady energy levels.
When your glucose is oscillating up and down like a roller coaster, this can cause feelings of fatigue. Additionally, unsteady glucose can impair sleep, which can further contribute to fatigue.
References:
1. Breymeyer, K. L., et al. (2016). Subjective mood and energy levels of healthy weight and overweight/obese healthy adults on high-and low-glycemic load experimental diets. Appetite 107: 253-259.