We all feel tired from time to time. During meditation or yoga practice, our mind and body may become very sluggish. We don’t want to move, we don’t want to concentrate; our will is sapped; we may be physically or psychically exhausted; we may be in pain or in extreme discomfort. If we mechanically proceed with the practice, we may sleepwalk through it, or space out entirely. This robotic activity is not skilful and little benefit will accrue to you if you persist in the practice while in this state.
In yoga this state is often termed tamasic – this is inertia, a dullness that steals the will to perform. The cure is rajas or activity. If we are meditating with eyes closed, we may need to open them and sit a little taller. If we are practicing yoga, we may need to add more vigour to the poses; do several vinyasas. 
However, what the body may need is rest.
We can tell only by really paying attention to what is happening. Are we sluggish because we have exhausted ourselves? If this is really the case, further effort will make the situation worse. If you are depleted from too much yang activity, rest. However, if you are just feeling a cycle of inertia, notice this and react accordingly to wake yourself up. Too much yin is not good. The cure is a bout of brisk yang activity; this can be mental or physical activity.