Pete, this is great to see. Last August I switched to primarily decaf when I became debilitatingly physically sick 36 hours after my last cup - meaning the withdrawal took over a day to hit.
The reason for this is because the half-life of caffeine (according to a doctor who studied this) is something like 8 hours, meaning if you drink cups at 8am, noon and 4pm, you go to bed with a cup's worth of caffeine in your system, and wake up needing to replenish. I was drinking 5+ cups a good number of days, and would purposely abstain here and there, but by putting so much of the drug into my body I was maybe accumulating growing amounts and so when I stopped, the half-life had to be cut down several times before I was lacking in it.