It's called Adrenal Fatigue. I take licorice root at the times of day when my cortisol is low. It's best to have a test for adrenal fatigue, the test I had was to take 4 saliva samples.
7-8am your cortisol levels should be high to wake you - mine are low so I get someone to wake me and give me a licorice capsule! Then 12 noon they should be high, my levels are high and this is when I usually wake up. Then 4pm they should be high, mine are low at this time and I usually feel drowsy and low energy, I take licorice at this time. Midnight they should be low to allow you to sleep, mine are not low enough so I can't sleep!
I also take adrenal dessicated glandular, 1 tablet a day. Siberian Ginseng is good for adrenal fatigue too, and vitamin C in high doses, vitamin B in high doses.
I takes a long time to get better, I'm still not fully well.
I searched for years into why I was so tired, do as much research as you can.
I have a nutritionist that diagnosed me and gave me treatment. A herbalist would help too. Most doctor's don't know much about adrenal fatigue and candida. They don't take you seriously when you tell them, even if you've had tests!
I see my doctor for hypothyroid. You may be hypothyroid for years, and it will not show up on blood tests. Mine finally showed up when I got really ill. It's taken years and many tests for it to show up, and I'm vey ill now.
Do a search on the internet for adrenal fatigue, there's a forum on curezone for it too. Look at candida and hypothyroid too.