Hello, welcome to health care magic. I am Dr Alok Sinha, MD Psychiatry from India. I will try my best to answer your query. I have carefully read your question and I understand your concern. I am glad to know that you have observed a very scientific phenomenon. Let me describe it for you. CPAP machine is used to treat a condition called obstructive sleep apnea. It delivers air at a higher pressure to pass through the airway and thus to overcome obstruction of pharynx. If a patient uses the machine he gets normal oxygen delivery to brain and a uninterrupted sleep. If machine is not used, as in your case, it causes reduced oxygen delivery, causing stress to brain. At time of stress our brain secretes excess of cortisol which causes diuresis (increased formation of urine). Secondly we all know that our body loses water through lungs during process of respiration. Since, respiration is regular and gas exchange is more effective at the level of lungs with the use of CPAP machine, more water is lost during respiration as vapour, so less water is excreted as urine. I hope it answers your question. I wish you a good health.