@creative_writer pretty much summed it up! @Jynx from what i've been told, caffeinated drinks and sugary drinks (or sugary/sweet foods in general) should be avoided on an empty stomach as it can spike cortisol levels and can contribute to feeling more stressed/heightened throughout the day. even sugary/sweet foods for breakfast/first meal can mess with blood-sugar levels (hence why desserts tend to be the last part of meals!) and make you more tired quickly.
basically when you wake up, your body natural starts releasing cortisol, but then having caffeine and sugary stuff spikes it more. i think having food in our stomach helps absorb some of that caffeine so that not too much cortisol is released. to be fair, some people can have caffeine/sugar first thing in the morning and live a long healthy energetic life - but if you have anxiety, chronic fatigue, high blood-sugar, etc., it may impact you a bit more. hope that makes sense!!