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I think I've seen @hippy mention doing that ahead of his 24 hr races.
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I have done but I'm VERY caffeine tolerant so I've never really noticed a difference.
I was talking to some sports scientists the other day (as you do) and they reported very different effects. One didn't like caffeine (weird tingles) and the other was more like me, could have lots and go straight to sleep.
In a 12hr, you shouldn't really be struggling with sleep so I would take caffeine for the whole event. You don't need to abstain in order to get the performance benefits. I normally keep it to 600mg/12hr when racing 24s although could quite easily take grams of it I don't want any gastric effects later in the race.
I'm doing ok on current gels and carb & electrolyte drinks at the moment. Bars are ok when I stop, but I nearly choke myself trying to chew food on the move.
I was thinking of not using anything caffeinated until around 9-10pm. Do you reckon it's worth staying away from caffeine for a little while beforehand, to get a greater benefit?