Biologically, the cause of lethargy is lack of haemoglobin. It can be one of the after-effects of an infection that has left the body weakened. Lethargy can also be the result of a psychosomatic disease.
entire cervical area
lack of energy
chronic dormant infection
The key to increasing your energy level is to improve your breathing. Try the following.
Breathe deeply and sigh. Force out-breaths to completely clean out the lungs. The best time for breathing exercises is first thing in the morning.
Practise Tantric exercises.