The antonym of energy is lethargy. Lethargy refers to a state of sluggishness, inactivity, or lack of energy. It is often associated with a feeling of tiredness or a lack of motivation to engage in physical or mental activities.
For example, after a long day of work, you might feel a sense of lethargy, making it difficult to muster the energy to do anything else. This term is commonly used to describe both physical and mental states where energy levels are significantly reduced.