He doesn't mention orexin specifically, but I'm sure you all will make the connection.
Think Twice: How the Gut's "Second Brain" Influences Mood and Well-Being
The emerging and surprising view of how the enteric nervous system in our bellies goes far beyond just processing the food we eat.
The second brain informs our state of mind in other more obscure ways, as well. "A big part of our emotions are probably influenced by the nerves in our gut," Mayer says.