Adenosine is a natural neuromodulator that regulates sleep, anxiety, and neurotropic receptors (in a different way than GABA or Serotonin). In many countries people use caffeine (from coffee, tea, chocolate, yerba mate, otc pills and energy drinks as an Adenosine inhibitor (antagonist) to delay sleep and promote energy, however this can negatively effect sleep and may worsen anxiety issues.  Adenosine agonists (stimulators) like d-Limonene however can not only positively effect sleep and stress/nervous reactions but also help reverse the side effects of caffeine directly.