It is no secret that the human body requires a decent amount of potassium to function properly, and a constant potassium deficiency can lead to chronic fatigue. That being said, if you perform a mild physical exercise that leaves you completely devoid of energy, potassium may be the culprit. Similarly, if you feel exhausted in the morning, even if you sleep sufficiently, you may want to check your potassium levels.
On the other hand, when your potassium level decreases, your heartbeat lowers and you are likely to faint. Of course, this may originate from multiple reasons, and is not quite common, but if you display these symptoms, call your doctor.