Why does sinus congestion or sinusitis cause fatigue?

Sinus cavities are like small water pockets in the head. Any air, dirt or dust is not inhaled directly by us. The dirt sticks in this mucus, hair in nostrils and only fresh oxygen goes into your body and lungs and so on. This is just one of uses of sinus. So as i said, these sinus cavities or water pockets gets inflamed or blocked when there is too much dust, allergy, due to cold or sinus then you will start breathing less oxygen (you wont notice the difference but you are breathing less). In most cases, after sometime it should become normal, however, if sinus is blocked and for a long time, less oxygen, less breathing, So the tiredness.