Eisai has launched its insomnia drug Dayvigo in Japan, the second in a new class of medicines after Merck & Co’s Belsomra, which made its debut there in 2014.

The FDA’s traditional last-minute flurry of approvals as the year heads to a close included Eisai’s Dayvigo, a new treatment for insomnia that is the first direct competition to Merck & Co’s ...