Extreme fatigue could be due to anemia, diabetes, hypothyroidism, or due to kidney or liver problems. Another possibility is
sleep disturbances (like
sleep apnea, imbalance of rapid eye and non rapid eye movement sleep etc.). You may sleep your customary 7-8 hours and yet feel fatigued. You need sleep studies to rule this out. Other possibilities are chronic fatigue syndrome, depression, low Vit b12, low iron, low calcium, sodium and potassium.