When I get solid in my training and the distances start to creep up I cut out all recreational caffeine (pop, some tea, etc...) and restrict myself to caffeine only for long training sessions as a boost.
This leads, always, to falling asleep; in meetings, at my desk in the middle of conversations with co-workers, on the couch before my kids, while reading a story to my daughter, while talking on the phone at home, in the car while in the parking lot.
In prepping for my next race I've cut out caffeine this past week, so far I've been lucky and only crashed out on the couch in the evening and spent the days yawning.
Sean
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