It is normal to feel a lack of energy, or to be tired or drowsy at times. This is called lethargy, and it is usually transient. A person may even act sluggish, listless, or uninterested. When lethargy continues, it may be a sign of a more serious problem.
Depression is the most common cause of persistent lethargy.
In cases of serious injury, infection, or disease, lethargy can worsen into a daze, or even unconsciousness. Be sure the lethargic person can answer basic questions such as the day or date, where he or she is, and commonly known facts. If not, get immediate medical attention. Call 911 if the person passes out. It is not good simply to treat lethargy with caffeine, drugs, or vitamins without a medical evaluation.