Fatigue is one of those little things patient of many possible causes and corresponding remedies--often with more than one cause and remedy needed. Sleepiness is a particular species of fatigue with probably fewer potential causes, but still too many possibilities to make treatment a one-size-fits-all sort of deal. Sleep apnea can be one of which one is unaware for some time unless one is tested. Poor sleep habits/prep for sleep can contribute (or room not dark enough, eating too close to bedtime, etc.). Emotional difficulties can too (e.g., stress, anxiety). Or digestive processing (e.g., IBS). Or a low grade chronic infection (in teeth, at an old injury site, in the intestines and so on) or allergies or sickness. Or hypothyroidism. Or nutritional deficiencies of various kinds (minerals and proteins have been mentioned, but there are other possibilities). Or excessive bleeding (women). Or over-exercising. Some say overexposure to various forms of radiation from techy stuff or high tension wires for some cause fatigue. Or growth spurts in the young. Or metal toxins (e.g., mercury, aluminum, cadmium, nickel). And so on.
In nutrition, enough and varied high quality probiotics might help (e.g., acidophilus and bifidus varieties)--e.g., in producing enough B vitamins. Ginseng (Siberian, Korean), Royal jelly, Rhodiola Rosea, the right kind of iron (for women). etc. Vitamin D3 deficiency can have varied ill-effect and may be worth a mention even if indirectly to the topic. Vitamin C collects at the adrenal glands, which should say something though usually one assumes sufficient intake. A mal-aligned spine can also contribute to organs not functioning at their best. Yes, moderate regular exercise not too close to bedtime can often help.
So much for the tip of the iceberg. Or part of the tip.