Extreme fatigue could be due to anemia, diabetes, hypothyroidism, or due to
kidney or liver problems. However since you had blood tests all these would have been ruled out. Hence the other possibilities are sleep disorders and inadequate sleep. You may appear to sleep the customary 7-8 hours and yet not get the required amount of sleep in the right pattern. Hence it is important to get sleep studies done. The other possibility is low Vit B12, low iron, low calcium, sodium and potassium.