My urine osmo varies from almost zero (close to being pure water) and high end of normal. I eventually figured out what was happening - two different types of polyuria, one central, DI-type and one driven by solutes, which I now think is probably mainly lactate, resulting from hyperlactaemia which in turn results from over-exertion. This, and my theory that my severe hyponatraemia in 2007 and 2010 was due to sodium loss in urine as sodium lactate (driven by hyperlactaemia), were supported by a test finding of hyponatraemia plus quite high urine sodium. My body should have been trying to correct the low blood sodium by reducing sodium loss in urine (natriuresis) but maybe the excess lactate made it impossible. Urine osmolality is largely due to sodium, ditto serum osmo, if I remember rightly.