What Causes Tiredness Throughout The Day And Lethargy?

hi, for several months now i have been tired all the time. i seem to hit a low point every day about 3pm. i have a lack of energy. in the evenings i tend to get light headed and sometimes feel as tho i am going to faint. i quite often have to have a sugary drink during the day for n energy boost. any idea what might be wrong. thanks