From my experience they can pull you over for any "reason" they think of. I remember getting pulled over at 2am once after being out on the town on a Saturday night and the reason they gave was "you were driving very cautiously sir". Of course I was f**king driving cautiously, id spotted the police car following me for the last half a mile, I was hardly likely to start driving like an idiot! They couldnt wait to breathalyze me but i hadnt been drinking at all so I was fine. They looked very dissapointed.
It varies drastically depending upon the person I think. I remember one night (long time ago now!) I was up until about 3 or 4am drinking tons of vodka with an ex girlfriend. I got about 3 hours sleep, woke up with the hangover from hell. I felt so sick I couldnt stomach eating any breakfast and was struggling not to chuck up. I cant remember why now, and its not something id ever do now, but I then decided to drive over a mates. On the way over to my mates, sods law, someone crashed into me (100% their fault as they were doing 60 in a 30), but the first thing the police do is breathalyze everyone. It was only then that the situation really hit me, until then I hadnt even considered I might be over the limit. However I suddenly realised that having been drinking from about 6pm until 4am, having hardly any sleep, no breakfast to "soak it up" etc, I must be well over the limit. All sorts of things flash through your head at that moment such as how are you going to cope with a driving ban, how will you get to work, will have to sell the car etc. Anyhow. I blew into the breathalyzer, struggling not to throw up over the officer, and it came up totally clean. It certainly taught me a lesson.
How to fake a breathalyser test by the sounds of it!