Sherlock and John have drifted apart somewhat after John's daughter was born, but John still visits Sherlock more or less regularly. One day, Sherlock asks John to please stay the night. John's reluctant not go back to his family, but eventually agrees and sleeps in his old room. He wakes up in the middle of the night and notices the door is ajar even though he closed it, and finds Sherlock sleeping on the floor right outside the door.

Eventually John understands that Sherlock is traumatized both from his time away and another occasion when he got caught and tortured while on a case alone and/or protecting John and his family, and suffers from insomnia and horrible nightmares and only manages to get a decent night's sleep if he can hear John breathing.

John realizes how much Sherlock needs him and helps him recover.

(Platonic relationship with pining Sherlock or slowly evolving Johnlock is fine, but please no John cheating on Mary.)