Though not mentioned in Desk Reference to the Diagnostic Criteria (which is quoted in ivan37's writeup)1 an oft-sited practical symptom of caffine overdose is a sense of impending doom.
This is the symptom which makes attempts to sleep so tortuous: everything you've left unfinished, everything you regret, everything you've been worrying about whirls through your drug-addled mind, and it seems these things are enough to break you, or destroy any sense of order left in your life.
1. Verbatim from Frequently Asked Questions about Caffeine, available at http://www.cs.unb.ca/~alopez-o/Coffee/caffaq.html, which is worth a read. The newest version is kept at http://coffeefaq.com/caffaq.html but there are annoying graphics and such there.