I awaken mornings to severe cramp in lower leg muscle. Does anyone know what causes this, I'm seventy and male

Do you smoke? It could be caudication which is a condition to do with circulation. You should see your GP about this, as it can possibly by as a coupkle of the othere postins said i.e. re. magnesium....salt.....but if it is the circulation then it is a heart condition which needs GP attention. I wonder if it is impairing your walking generally? if so then i am sure it is caudication. See your GP monday.