When I do meditate I yawn a lot and feel tired.
Am I over exerting myself?
—Edward, South Africa
This feeling of tiredness may just be your body relaxing — in a new way — and the body needs to adjust to that feeling. At the same time your meditations will take all your focus and concentration which can, at first, be very tiring — -interesting duality.
You may want to have shorter but very focused, concentrated meditations for awhile.
I hope this helps, let me know how you do with this idea.
Joy to you,