What we talk about as laziness is the laziness of attention (consciousness).
When developing the skill of meditation, we have to learn to balance many elements in ourselves. Just as when you are balancing anything, there is often a swing from one side to another until equilibrium is achieved. The swing between drowsiness and hyperactivity is one such pendulum. Again, this is primarily in relation to consciousness, not physicality. Physically, we want to retain relaxation, but not laziness.