It's 11 p.m. on a Friday as Sarah Britton and her friends pass through the doors of the Aspen Hill Club. The beep of a nearby wristwatch signals not the end of the night, but merely the beginning: For the next six hours, the Rockville seventh-grader and about 200 Montgomery County Girl Scouts will have full run of the health club as part of an annual "lock-in" event.

Similar lock-in events happen nearly every weekend throughout the country. With kids trying to pack in as many activities as possible, some organizers find it easier to get people together in the middle of the night for long, uninterrupted sessions. Some scout troops work on projects or medals. (One scout used the Aspen Hill event to work on a leadership project.) But most kids get together just for the fun of it.

For the first few hours at the health club, the girls show no signs of slowing down. Many race to try out the various pieces of exercise equipment -- treadmills, bikes and step machines -- as if they were theme park rides. "If they're not doing another activity, they're on those things like Energizer bunnies," said Lyndi Lahl, co-leader of Troop 1065, and one of the adult chaperones.

Down the hall are sounds of high-energy racquetball games. And volleyball and basketball; aerobics and hip-hop classes; swimming; and lots of laughing, talking and snacking.

Time begins to take a toll after a few hours, and some of the scouts drop in a corner among sleeping bags and pillows. But, 12 of the 13 Energized bunnies from Sarah's troop keep going . . . seemingly intent on getting the most for their $10 fee. Besides, they didn't even bring sleeping bags.

"I can just sleep the next day," Sarah explained later.

Shadowland Laser Adventure Center in Gaithersburg hosts several lock-in events each year. "It's geared toward teens, to give them a chance to hang out," said Shadowland's Phil Vongsavang. "Church groups have sleepovers in a church. We get away from that, with a little less structure."

The price may seem steep -- $1,449. But groups have the run of the facility for seven hours, starting at midnight. And with the extended time, the players often try some of the 100 variations of the basic, 15-minute, $7.25 laser-tag game. Thirty people can play at a time; the rest wait their turn by snacking and watching movies.

The National Cathedral hosts lock-ins in the spring and fall. In addition to games and movies, the program usually includes some religious discussion -- and some sleep.

Kids arrive at 6:30 Friday night for dinner and stay until 8:30 a.m. Saturday. Activities include a tour of the crypt, a demonstration of the massive organ, and a tower climb. "They're out in the atmosphere climbing the tower," said the cathedral's Paul Canady. "There are spectacular views of the city from 16 to 20 stories up."

Some kids go to sleep early, finding quiet spots in pews or in the hallway. Others use the freedom of no bedtime to test their endurance.

Thirteen-year-old Mallette Asmuth of the District has attended several of the church lock-in/sleepovers. "I love it," said the seventh-grader. "I love staying up late and socializing. The conversations get different. I'm the sort of person who usually stays up late. I'm a party person."

And at a lock-in, if it's late, the party may be just beginning.

-- Scott Moore