I don't know if it's psychosomatic or not, but I find that how much caffeine I'm ingesting on a regular basis affects how much caffeine affects me. When I was in high school and college, I used to drink iced tea as my regular drink (caffeinated, although tea doesn't have a lot in it). I would also drink some amount of Coke, which has more than tea but nowhere near coffee. Because I was regularly drinking a low amount of caffeine, my body was used to it. I could have a Coke or iced tea late at night, no problem.
Later I switched to decaf tea (which still has caffeine but it's *really* minimal), and for a while I wasn't drinking any soda. So I was having almost no caffeine on a regular basis. I then found that when I did have a Coke, I could really feel the effect. Suddenly soda would keep me up at night when it never had before.
Now I've been drinking Coke a lot more, and my system seems to be somewhat confused. I have noticed that if I drink a Coke and then try to take a nap soon afterwards, I sometimes find that I doze lightly but I never seem to fall asleep, and I just keep thinking about things. I don't feel like my heart is racing or like I'm all hyped up, I just don't seem to be able to drop all the way off to sleep.
On the subject of food dictators, my husband isn't as picky an eater as I am, but he's generally a less healthy one. He has the tendency to binge on a food... eating an entire bag of M&Ms, things like that. I'll eat plenty of sweets, but generally a lot more moderately. But he'll also go through periods when he tries to eat really healthy. And instead of just, say, not eating a ton of candy or whatever, he'll start eating some sort of spinach-and-egg omelet and pea soup and asking me to buy kale and stuff like that. And then he really wants me to eat this sort of thing with him, and I feel bad for not doing it too and discouraging him. But... I don't like spinach. Or kale. Or runny eggs. Or any of the other stuff recommended through this exercise program or whatever it is. I might try to eat the same foods that I normally eat, but maybe try to eat a little less junk and a little more good stuff, but that's about the only change I'm willing to make. And then the health kick he's on doesn't last and he's back to regular diet, and I'm never quite sure if it's my fault for not being more supportive or if it was pointless for me to join in because he would have dropped it anyway. Oy. He's not really being a food dictator as he isn't trying to force me or make me feel guilty, but it's just sort of frustrating.