Alcohol can also cause a rise in noradrenaline and this is known as our fight of flight hormone. If we don't have normal levels of that our brain isn't able to lay down memories in the same way. People who are more shy had much higher levels of anxiety the following day than people who weren't shy. It could also be a psychological effect people who are more Shy might use alcohol to numb some of those feelings of social anxiety when you're going into a situation.