Why is coffee never quite a good enough substitute for sleep?

Because it doesn't get rid of your body's need for sleep--it just disguises it.

sjg102379 hit it. Coffee is a stimulant that triggers a bunch of chemical messages that are also triggered when you're naturally feeling alert, but there's a limit to how loud those messages are and how long they last. Once they wear off, you're brain still needs to turn out the lights for a while. Also, there have been a number of studies recently showing that you actually need to sleep in order to learn. If you don't sleep your brain can't get a handle on the new information it was given.

And not just sleep, but REM sleep. If your body is denied REM sleep for an extended length of time (can't remember exactly), your brain begins to malfunction. It's scary.

Your body has a built-in biological clock that must, sooner or later, be satisfied. Back when I was a corrections officer, I would frequently work multiple shifts consecutively. I generally found I would start to hallucinate at 42-44 hours without sleep, regardless of the caffeine and nicotine I was cramming in. They did an interesing study a few years back where a grad student volunteered to spend a year underground without any clues as to the passage of time. They found after several months that her biological clock, without any clues as to what time it was, settled in at a circadian "day" of 23 hours.

physiologically speaking, coffee has the exact same effect on our bodies as stress does. it can help stave off sleep but it in no one gives us the same benefits that sleeping does to our bodies

It dehydrates you like crazy. It’s great for a quick perk, but to replace sleep… not so much. I don’t know if dehydration and energy have anything to do with each other, but for me when I’m dehydrated I tend to be more tired.