I find that acidic juices such as Orange or Ananas juice cleans out most of the gunk that builds up overnight and gives you a short boost through the sugar in them. The sugar can usually cover until the coffee after begins to work so you don't get that period of slouch after drinking coffee before it works (drinking something warm tends to make drowsy too so you possibly get more tired after drinking coffee). Skipping/preventing this short moment of weakness improves on the day a lot.
Not to my knowledge, the acidity of the juice helps a lot to get more awake very quickly, pure sugar itself doesn't help that much to get the continous effect.
