title_body: How much caffeine is lost after 24 hours?
upvoted_answer: Caffeine does not break down (at any appreciable rate) when left alone in a drink. It requires the enzymes in your body to break it down, so when you take the drink out of the fridge, it'll be just as caffeinated as when you put it in.
  
  However, your drink will still be stale, because there are plenty of other organic compounds in the coffee that will break down over that time, affecting the taste. Also, assuming you reheat the coffee, the reheating will have further effects, making it taste very bitter.
  
  Either make yourself a smaller coffee (16oz is a lot!) or drink it all when it's fresh.