Think of it like driving. When you first learn to drive everything is conscious 'put foot on clutch, press, put hand on gear stick and move it up then right, take foot off clutch and put it back on accelerator'. As you become more practised, you just think 'time to change gear' (sub-conscious) and eventually you change up and down unconsciously. Muscle memory means that you can do the things that at the beginning were really difficult to remember. If you are tired, your driving standard can be compromised by way of reaction times, concentration, etc. Drinking alcohol obviously has a very serious effect on someone's ability to drive safely.
Your brain is having to relearn bladder control and at the minute you are probably at the sub-conscious stage. You don't have to think about it in small steps to be able to do it, but it still requires some conscious effort. When you are tired or have a drink, your concentration level drops.