Some days, you're full of energy, on others... not so much. Even so, if you find yourself straddling the lower end of this scale more often then not, it might be time to contemplate a few lifestyle changes. And they don't have to be that radical, as this video from AsapSCIENCE explains.
Putting aside medical conditions, such as sleep apnea or diabetes, AsapSCIENCE goes over a bunch of options you can try that are well within your control.
Depending on your lifestyle, squeezing even a little exercise into your routine can boost your energy levels, as can staying hydrated.
You also don't have to give up caffeine, however, if all else fails it might be worth skipping it for a few weeks to see if it improves your sleep quality. Alternatively, avoid getting your hit later in the day -- try and survive on single morning shot if you can.
Of course, if you suffer from prolonged fatigue and the above suggestions have little to no effect, best to give the doctor a visit to rule out anything more serious. Even if you get the all-clear, your GP is in the best position to provide recommendations relevant to you.