Someone who functions better in the morning, as compared to later in the day.
This is an American phrase from the latter half of the 20th century. The separation of people into owls and larks, i.e. those who go to bed and get up late, as opposed to those who go to bed and get up early, has led to their description as either a night owl or a morning person. The earliest citation I can find of morning person is a reference to 'morning-personality' in an advertising piece from the Indiana Evening Gazette, in January 1960. The article encouraged the purchase of instant mashed potatoes and was headed 'Pillbury Has Potatoes In Easy Package Form':
"What kind of 'morning personality' are you? Are you the kind of gal who has to drag herself down to the kitchen in housecoat and flapping slippers, to get her family's breakfast?"
The earliest citation that explicitly mentions 'morning person' is from the Minnesota newspaper The Evening Tribune, February 1964:
"Your morning person sprints through his task until noon, but after lunch he sags listlessly and can't wait for quitting time."