Remember how tight you were with your besties in high school? You shared your deepest darkest secrets, you stood by one other through all sorts of shenanigans, and you'd do anything for each other.
It turns out those friendships have a far more lasting effect than we realised.
A study published in Child Development recently found that the close relationships teens create while in adolescence can have a positive impact on their mental health for years afterwards.
The study followed 169 people for 10 years of their lives, starting when they were 15 years old. They were asked to bring in their closest friends for one-on-one interviews at regular intervals.
"[They were asked] how much trust there is, how good communication is and how alienated they feel in the relationship," said Rachel Narr, lead author on the study and a doctoral student in psychology at the University of Virginia, to NPR.
Each participant also filled out a questionnaire each year to assess their levels of anxiety, depression and self worth. Rachel says that throughout the study, the positive effects of strong friendships became obvious.
"These teens tend to be open with one another about difficult topics, and they're more engaged with one another and helping the other person and connection with the other person," she said.
Rachel said when she watched videos of the teens asking their friends for support or advice when talking through a problem made in the early years of the study, she could easily tell which relationships were strong.
"These teens tend to be open with one another about difficult topics, and they're more engaged with one another and helping the other person and connecting with the other person," she said.
As the study participants became adults, those with close relationships were better off emotionally. They showed less symptoms of depression and anxiety, and better self worth, at 25 than they did when they were 15 or 16.
"It surprised me how much better they were doing," Rachel said.
Rachel and her colleagues also looked at how popular the teens were at school to see if that had any correlation with their mental health and happiness, but they found popularity was immaterial. Strong friendships were the one defining factor.
So the message is clear: if you want your teen to grow up to be a mentally healthy adult, it could help to foster strong friendships now. So saying yes to that sleepover might be a good idea.