The Incense of Spring Sleep (春眠香 Shunmin ka?) (translated as Shun Min Ko) is an incense which Happosai learned about from his grimoire and made to try and put Ranma Saotome to sleep for the entire spring as payback. This was for turning his potent sleep incense against him and beating him up in the manga or distracting him so that the pursuing female students at Furinkan High School can beat him up in the OVA. However, while trying to make Ranma smell the incense Happosai makes Akane Tendo inhale it instead, causing her to fall to its effects after an unknown time period.
The incense is meant for those who cannot sleep due to their lives being in constant danger. It allows them to sleep while still being able to defend themselves from assassins and potential threats. However, should the incense be used in a time of peace the user will go on an uncontrollable rampage. In both versions Happosai failed to read the entire description of the incense's effects as he was too focused on the primary effect of putting the user to sleep through the spring. As the incense was used on Akane during a time of peace she began to go on a rampage through Furinkan and severely damaged the Cat Café, although in the manga she does not get out of the classroom. While affected by the incense Akane's reactions to her vivid dreams affected how her body behaved the real world, such as attacking Ranma for two-timing with Shampoo and Ukyo Kuonji in one of her dreams.
There seem to be several ways to counter the incense. In the manga Akane was awoken by the smell of mosquito repellent. In the anime several alternative methods were added to possibly wake her up. These include using a Spicy Incense (which acts similar to the mosquito repellent in the manga) which, according to Cologne, is able to wake anyone who smells it, regardless of how asleep they are. However, in the anime Akane was eventually awoken by Kasumi, who, lacking any sense of bloodlust or evil thoughts about her, was able to get close enough to Akane to wake her.