One example is low epinephrine and norepinephrine. This can cause low energy, low mood, lack of focus and direction. Another example is low serotonin, if you have low serotonin, you may not be getting good deep sleep and that can ultimately lead to fatigue.
Each person’s neurotransmitters are different, some high, some low, some normal. Deficiencies or excess levels can also be due to nutrient deficiencies in amino acids, vitamins or minerals.