It makes sense. It's pretty much impossible for you're mind to race if you've got a throbbing headache. The hangover forces you to be "in the now" and not in your head. It forces you to focus in this very moment, which prevents your mind from wandering into the various states that trigger a high or low.

For example, I've been dealing with the flu for a week. It's kind of the same thing. I've been sleeping and making herbal teas pretty much 24/7. The sickness forces my mind to dealing with the sickness. As a result I haven't had a chance to slip into a manic or depressed state. I'm too busy buying Kleenex, cooking foods with spicy ingredients, and finding the right fruits to reduce my pain. The physical symptoms are all-consuming of your energy.

I'm in the last states of the cold, and I'm starting to notice a mild depression setting in. This is because my throbbing sinus pain has finally stopped. But now I have energy to think about how I wasted a week sleeping and coughing, instead of looking for a job, and it's making me depressed.

I've noticed the highs and lows (at least for me) have a lot to do with how you spend your mental energy. When I'm consumed with a task (sickness, hangover, at project, etc) my mind doesn't have the energy resources to be manic or depressed. I'm just focused instead.