There are many reasons why you may lose urine, but the main reason is that your bladder cannot store your urine for the whole night. The next is that: 1. your bladder is not large enough to store your urine for the whole night; 2. although your bladder is large enough, spasms occur before it is full, causing it to pass urine; 3. you lack a chemical called antidiuretic hormone, which tells your kidneys to produce less urine during the night. So even if your bladder is of normal size, lacking this substance, the bladder cannot store this extra urine; 4. constipation, if you don’t have frequent bowel movements, feces can build up so that your bladder has a hard time holding the urine in and staying in the bladder; 5. drinks.