Energy arises when one has a clearcut direction. One knows exactly where one is going and keeps at it. But when the mind has no clear concept of what it's actually trying to accomplish other than staying alive physically, not much energy is produced. It's not fascinating or interesting and the subconscious mind knows already that it's a lost cause. Nobody can survive. To use one's strength and direction just for survival is not a fruitful undertaking and real energy will not arise. On the contrary, one feels bogged down and oppressed by it.
The Buddha compared sloth and torpor with being in prison. When one is in prison in a little cell, there's nothing one can do until somebody opens the door. When the mind is beset by lethargy and drowsiness (lethargy is in the body and drowsiness is in the mind) it is imprisoned to the extent that one can only just rouse up enough energy to do the most necessary things.
Most people don't know and don't accept that meditation is a necessity and so the mind easily gives up. One has to be clear about the efficacy of meditation. It's not only necessary to eat, sleep, wash and dress. These are automatic survival techniques, and don't need a lot of energy. They are instinctive. But meditation needs energy and that can only be aroused if one knows the importance of it, if the mind is quite clear that this is what one really must do.
[...from Five Hindrances... the 5th chapter in Being Nobody, Going Nowhere by Ayya Khema, pg. 75]