The night before is the deciding factor when and determines the kind of morning after you'll have. This is especially important when the morning after just so happens to be a big day (job interview, exam, wedding et al.) Sleep is highly recommended the night before any of these occasions; though nerves and anxiety makes falling asleep highly unlikely.
On some nights, like Christmas Eve an New Year's Eve, staying up late in anticipation of the upcoming big day is encouraged. You are allowed to stay up late singing, children stay up past their bedtimes listening to stories from Christmases and New Years Eves gone by and everybody has a good time. The next day -- the big day -- people wake up mostly functional and still excited and go on with the day's festivities.
What to do then when your big day requires that you are at your full mental and physical capacity? That your mind is sharp and reflexes working? Here are a few tips that have worked for me in the past.
Go to sleep
I know, you're sitting there thinking "easy for you to say, you don't have the butterflies I'm experiences about X." I recommend that you follow your nightly routine and when your bedtime comes, get into bed and switch off the lights. When the mind calms down, the body will follow suit and do what it normally does at that time and in that environment.
Let's backtrack a bit...
Before you can be fit to get into bed you have to remind yourself that you've done all you can to prepare for the big day; that you've studied enough, have learnt your vows and dance by heart, that you are prepared for the interview, whatever preparation that's required, you have done it. This is but the first step.
When you have accepted that the big day is coming and you have done your best to prepare for it try to calm yourself. I've found that a long hot bath calms my thoughts best and leaves my body longing for my bed. Light a few lavender-scented candles, make a cup of chamomile tea and soak all your stress away. Lavender and chamomile make for a wonderful pre-bed ritual as they soothe the body. The Late Bloomer discusses the benefits of both here and here.
Even when the body is ready to sleep the mind might not be. Write down the feelings and anxieties you might be having and get them out out of your head and then Go to Sleep.
Sealy takes care our our nights before, which is why we have the best Morning Afters we can.