Hello, When you sleep, your muscles lose tone and fluid tends to pool along with your back. Stretching helps to massage fluid gently back into the normal position. Also, your muscles protect themselves from over-extension by inhibiting the nerve impulses as they approach their limit. Causes are lack of physical movement lead to the urge to stretch, dehydration and chemical response in the brain may lead to an urge to stretch. Until examination is done it is difficult to say what it is. Drink plenty of liquids and tender coconut water, eat green leafy vegetables. Get it done Ra factor, serum electrolytes and haemoglobin status then consult your doctor he will examine and treat you accordingly. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Penchila Prasad Kandikattu , Internal Medicine Specialist