"Space happy" was an informal term used to describe unusual behavior caused by long term space missions.
In 2266, Yeoman Janice Rand speculated that Crewman Green was "space happy" when he displayed strange behavior toward her. In actual fact, Green had been killed and replaced by the M-113 creature, whose actions were symptomatic of its hunger as it hunted for salt. (TOS: "The Man Trap")
In 2267, Captain Kirk suspected that Nomad was "space happy" for believing him to be its creator. (TOS: "The Changeling")
In 2373, after Damar suggested the irrational and impractical idea of shelling Ty'Gokor with a single Bird-of-Prey, Odo suggested to Damar that "You should ask Dukat for some shore leave. I think you've been in space too long." (DS9: "Apocalypse Rising")