What disease or illness should I look into if I feel tired, sleepy all the times, muscular and joint sored, decrease in memory and concentration?
Sleeping enough? There is a long list of possible causes, too many for 400 letters. We need 8 hours of sleep, more or less. If you are averaging 6 or less that may be part of the problem. Part only. Go see your doctor. Thats why we went to medical school.
Fatigue. The symptoms you are listing can be caused by a host of conditions or may be linked to one condition. The first thing that come to mind is perhaps a thyroid issue or rheumatoid issue. You should see your family physician to get some labs to really see what is going on.