It doesn't matter how much sleep I get, if I've been sat in the same place for more than an hour, I become lethargic and sleepy. This is all the more pronounced mid-afternoon. From anecdotal evidence, it seems to be a common experience.
Stimulants like caffeine and sugar give you a temporary energy boost, but the subsequent 'crash' leaves you feeling sick and even more sluggish.
The only solution I've found is to move about a bit. This can involve a spot of washing up, a walk around the block, or, my personal favourite, a few rounds of bowling or tennis on the Nintendo Wii (what do you mean you haven't got a Nintendo Wii?).
10 minutes of mild exercise will usually get your circulation going enough to allow you to return to your sedentary chores refreshed and energised.
If you're still feeling tired then a timed nap can do wonders (yes, I know, I know, tricky in a working situation). Set your alarm clock/ phone/ watch to wake you up in 10-15 minutes (any longer than this and you'll enter deep sleep, which makes it much more difficult to wake up).
Be prepared to feel rubbish when the alarm does go off, but get up anyway and within 10 minutes you should be feeling a lot better.
If you're still feeling drowsy, you might be ill. Or genetically bone-idle.
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