I was feeling the same a while ago and took a turn for the worse one evening when I nearly fainted and was shaking like a shi**ing dog. Tests in hospital proved that all was well, heart, vital organs etc and blood. I followed it up with a visit to the doctor, and after a chat, the reason became apparent...Some days when office bound I drank up to 8-9 cups of coffee a day, I skipped lunch, just having a banana and an apple, though I thought the good breakfast I was having was making up for it. But, the biggest shock was that I NEVER drank water through the day, in fact I never drank water on it's own at all thinking I was getting enough from other sources! Now, the good breakfast is still being eaten, porridge, raisins, some fruit and a drizzle of honey, that sets me up nicely. As I have an active outdoor job in the countryside and prefer to walk rather than drive when I can, I always take a bottle of water with me now and go through at least 1.5-2 lts of water. Dinner is a healthy snack, a sandwich, soup or a couple of boiled eggs with a round of bread and a bit of fruit after. No more tiredness for me now. I've slowly cut out the caffiene and introduced decaf over time. It seems to have worked for me and even the after work naps have stopped too.