I find that when I'm tired and not getting enough sleep I tend to go straight for high fat, high sugar foods (e.g. chocolate) thinking that they'll give me the energy I need. In actual fact, they tend to give me a surge of energy and then leave me lower than before. They also tend to affect my moods in a negative way, so I try to avoid them.
I find that the best foods to eat when I'm low in energy are foods that are fresh and full of vitamins, minerals and fibre. For example, I try to eat more fruit, salads, nuts, seeds and fresh vegetables. These foods keep me feeling light and give me enough sustained energy to keep me going.
Given the description you've included, I'm wondering if you could possibly be deficient in a vitamin like iron, vitamin B12 or folate. Deficiencies in these vitamins can leave you feeling tired, despite getting a good night's sleep. The best way to find out if you are deficient is to see your doctor and get a whole work up. Always get checked out before you start supplementing as having too much of some vitamins can also be harmful.
The other reason why you might be tired despite getting 8 hours of sleep is that you might be having restless sleep. Are you overweight? Is there any history of sleep apnoea/sleep walking in your family? I'd suggest you talk to your doctor about all of this as it is only with a proper sit-down chat and examination that you'll be able to get down to the root of the cause.