I have always been one to take evening naps since middle school. For the past 5 years I have had an extremely difficult time staying awake during the day. It doesn't matter if I go to bed and get 8 hours of sleep or 4 hours of sleep, I am still tired. For the past two years I have been getting at least 30 minutes of exercise daily. Usually walking 2 miles.
Nothing seems to work.
I have even had a glucose tolerance test and it was normal. Most likely a sleep disorder. Have your physician check your thyroid function. If it's normal, see a sleep specialist and spend a night in a sleep lab. There are several common problems, usually easily corrected which will lead you to be tired all day.