In the musical “Grease,” a tightknit band female high school friends, known as the Pink Ladies, have a lot of fun after dark.

There are slumber parties and late-night milkshakes and obsessing over a crush when they should be sleeping — all in the name of capturing the hearts of the handsome bad-boys greasers called the T-Birds.

That may sound like good times, but when young women go in for late nights too often, they end up sacrificing their health.

A new study published in JAMA Pediatrics finds that teen girls who usually go to bed late (no matter how much sleep they get) and have different sleep schedules on weekends compared with weekdays are more likely to gain weight than girls the same age who go to bed earlier and don't blow it out on weekends.

When you mess with your body clock, you can disrupt hormones such as insulin, leptin, and ghrelin — all of which are important for maintaining a healthy weight.

Each hour difference between weekday and weekend schedules was associated with a measurably larger waistline and higher body-fat composition for girls.

Excess weight in teen years is associated with everything from Type 2 diabetes to depression, joint problems, and premature heart disease.

So tell your teenage daughter about this and other research showing that night owls are 88% more likely to have emotional and behavioral problems than other teens.

Then help her get at least nine hours of sleep a night — every night.