You could be working to hard.

You're not getting enough sleep as you mentioned in another question, working every day ,yet not having a good night sleep might be the main reason for you feeling tired among other reasons as well,such as nutrition, lack of exercise, nicotine or caffeine intake. Check your life style in general and try some adjustments for your own good :)

I suggest that you have a blood test to check your iron levels. A lack of iron, found in red meat and leafy green vegetables makes you feel tired. Perhaps you don't get enough sleep?

Are you eating well?