Dr. Hibberd's answer:
 

Persistent fatigue is not something you should ignore. See your physician for an evaluation to be sure this is not related to a physical problem such as heart disease, a thyroid disorder, or diabetes. Other possibilities include depression or an untreated infection or even cancer.
 

By paying attention to changes in your health (emotional and physical) you can take advantage of the wonderful options available today for early disease diagnosis and management. If no cause is evident, remember that our biological clocks are light dependent and that sleep disruptions will occur when we travel through different time zones, not matter how much we rest during the day.
 

Try to plan your day to be sure you have sufficient exposure to light during your waking hours. Avoid overeating, be sure to maintain some daily exercise, and try to avoid napping, which can affect how well you sleep at night. If you must take an afternoon snooze, limit it to under two hours — ideally between 2 and 4 p.m.