[flocciput]

I've found seltzer makes me sleepy or fatigued for some reason. Everyone I tell this to looks at me like I'm nuts but I swear it's real. I think it has something to do with the brain's expectation of calories and the subsequent lack.

[MrMcCall]

I get sleepy if I have too many vitamins/minerals (eg: a whole PowerBar or entire Orgain drink), so maybe it's the minerals, perhaps Magnesium?

[Magnesium helps my wife sleep better, but she can only take it at night or it makes her sleepy. I take half in the morning, and I think it helps because we're likely not getting enough in our mineral-depleted foods.]

[deinonychus]

I can imagine that. If I drink a lot of diet soda, I get a bit wired but also tired in a weird way. It's like my body expects some sugar to fuel the caffeine's stimulation.