Remus had really bad insomnia while at school and they would often spend hours awake at night in the common room. It was then that they used a quick spell to copy out their notes for their friends. After all, with all they did for them it only seemed fair.
They never told anyone about the insomnia it would just worry everyone. And even when they did sleep they would just have nightmares about the full moon that changed everything. How they had asked their dad to check for monsters 'just in case'. But 'big four year olds don't need that anymore, do they?' And 'I guess not...' Except there was that night. And that's why Remus can't sleep if there's a gap under the bed between the bottom of the bed and the floor.
Every morning Sirius would wake up first (contrary to popular belief) and carry Remus back upstairs once they had fallen asleep on the sofa at 4:30am. And Sirius would have a go at anyone who thought it would be funny to wake Remus up. Sirius never told Remus that it was him who carried them upstairs every morning without fail.
Except one morning Remus wasn't there when Sirius crept downstairs at 6:03am.
Remus didn't appear until lunch of that day (it was a Saturday) when they proudly showed Sirius, James and Peter the first draft of what would later be known as the marauders map.
In all the hours spent awake Remus had been making detailed plans of the school, using maps from books and midnight walks.