First thing, think really hard about whether your observations are worth bringing up. You have only spent one night with this person, so do you really know him well enough to sabotage his application? I'll assume there was alcohol involved, so are you sure that your perceptions were accurate and that you were seeing how he would behave in a professional environment? Alcohol aside, are you sure his bar/bedroom personality is the same as his work personality? Is it possible your opinion of him is biased because of how the night/morning went?
Clearly I think that you should most likely not say anything about this. However, let's say that he did or said something egregious that you think is really worth bringing up. Go to the hiring manager or someone else you trust with influence and be honest. Tell them you met the candidate in a bar the night before not realizing who he was, and what interactions you had that made you think he wouldn't fit at the office. Make sure to plan out ahead of time what specific things you want to say about the candidate. If you think your arguments will be hard to justify to the manager, then maybe it's best to keep them to yourself.
There's no need to mention that you went home together, but it's also reasonable to ask the manager not to say who shared this information. Be clear that you've only met the candidate this once and whether there was alcohol involved. Also be clear that you will not hold it against the manager or the candidate if he is hired and that you will always behave professionally. Your motive here is making sure the hiring manager has all of the information, no to dissuade them from hiring the candidate.