First off, I’m assuming you’re getting a sensible amount of sleep every night? Around eight hours a night is a good ball park figure. But it’s not just the length of time that you are sleeping that matters, it is also the quality of sleep you have. As they say ‘it’s quality over quantity’.
And by quality of sleep I mean how deeply you are sleeping. It may be that you are not getting enough deep sleep and that your sleep pattern is very light. Quality of sleep can be negatively affected by snoring and problems with your breathing while you’re asleep such as sleep apnea. Tackling this could solve the situation.
But if you do think that you sleep enough and well enough (as in you feel fine first thing in the morning) then you may have an underlying condition.
Mental Health: Anxiety and Depression
Anxiety and/or depression can dramatically affect your energy levels, amongst other things.
Have you been tested for anaemia (that’s when you don't have enough red blood cells)? This can make you feel exhausted, however much sleep you've had.
It may be Hypothyroidism, or "sluggish thyroid,” which is a common problem, especially in women.
Diet, Medication and Vitamins
Are you taking medication for any other ailments? Some medicines can disturb sleep and cause daytime fatigue.
Sometimes instigating a change in diet with foods high in vitamins and minerals (especially iron) and low in carbohydrates and saturated fat can help tackle sluggish feelings. This is particularly true when combined with regular daily exercise.
To be perfectly honest, there is no guaranteed correct answer. As you can see, there are at least a dozen reasons you may be tired all the time.
It could be lifestyle choices or medical conditions, or it could be due to stress or emotional issues. Everyone is different and may feel tiredness for different reasons, so the cause may be unique to you.
If you are really concerned about this, and if it is affecting your quality of life - which it must be, to some degree, or else you wouldn't be asking this question - then I implore you to speak to your local doctor in person, and see what course of action he or she suggests.