Berries questions are great and may lead you to an answer. Hopefully you can talk to a professional about what you're experiencing.

Several years ago a doctor told me some studies (which?) suggested the afternoon sun served as a natural sleep cue for many people, this without regard to the size, contents, or precise timing of the midday meal. In your case, however, the effect seems exaggerated.

I really hope you can find a way to overcome this, Shame. Fatigue is a bummer.