“You want to seem as cool as possible so you will post something at 2 in the morning, to just be like, ‘Oh, I’m part of this cool-kid group,’ ” one friend said, before Ms. Fagbenle added, “My friends and I see the same thing down our newsfeeds, posts about #breakingnight, also known as #notsleepingatall and #vamping.”

(There are other, sexual definitions for vamping, but those are unrelated.)

The word has even gotten the attention of academics.

“Social media is about having agency over your own life and vamping is one way to recapture that,” said Alice Marwick, an assistant professor at Fordham University who studies the Internet and society.

Danah Boyd, a scholar and senior researcher at Microsoft Research, who recently wrote the book “It’s Complicated: The Social Lives of Networked Teens,” offered two reasons for the phenomenon. First, teenagers have a desire to connect and the solitude of night allows for intimate conversation. Second, they are reacting to overbooked schedules packed with sports, music lessons and homework that give them less free time to pursue personal interests.

“Parents think they are doing good,” Ms. Boyd said of teenagers’ busy schedules. “But hanging out is where young people begin to understand social dynamics.” She added, “Because of the restrictions placed on them, very few interactions are unstructured until their parents go to bed.”

There may be another reason, as timeless as high school dances and after-school clubs: peer pressure. A mother of a 13-year-old girl in Seattle (who requested anonymity so as not to embarrass her daughter) said her daughter has difficulty untethering from her social group after dark; she has often caught the girl chatting in the wee hours with at least four friends on Kik, a mobile messaging system.