After awakening in the morning attempting to get out of bed hampered by extreme pain and cramping in upper thigh and back of legs,after a while of pushing through pain to become mobilized pain level reduces and is only relievd when laying horizontal with legs elevated.
I take you have no lower back pain and have little or no trouble for the rest of the day. My first thought is a mineral deficiency, less likely it's true in the middle of a Oz winter.
Are you taking any meds? Water pills? High blood pressure medication? Talk to your pharmacist. It could be a potassium deficiency.
The next step is to test the reflexes and look for neurological changes.
A general rule is to contract the antagonist muscle: if the hamstring is spasming in the left leg, then contract the left quad against the other leg by hooking your left foot behind the opposite right ankle and extending the left knee - that usually forces the left hamstring to relax.
Jumping out of bed has a similar effect - forces you to contract the quad.
Elevating your legs too is contracting the quad.
There are other central canal causes that are very unlikely.
I hope this contributes.
Dr. Barrie Lewis