I've been rather silly today and stayed at the PC for about 9 hours doing some very important work.?

I know it's self-inflicted, but I now feel very strange - almost indescribable. A bit like being 'over-alert', but also 'flickery', and 'wired'. Anyone experienced this and know the quickest way to get back to normal? It's weird because as someone is talking to me, it's taking me a split second to register what is being said!