Where do you put your tongue during medidation? Also, do you practice lying down, sitting, standing up?
Posture can affect both the tension in your body and the ability for qi to flow freely which can cause you to feel more lethargic.
If you view qi as bioelecticitry and the body as having different circuits, placing the tongue on the roof of the mouth completes the circuit and allows for smooth flowing. If the tongue touches the teeth, this as a sort of grounding effect, and this energy leak can make you feel tired as well.
Hope this helps,