"I'm sure you will agree that we all must be considerate and respectful of our neighbours, we don't want to be sharing your intimate and private moments with you."
The neighbour then asked if the man could "please" keep the noise down at night and be mindful that sound travels.
Cunningham, of Glasgow, Scotland, told local publication Daily Record he couldn't stop laughing when he found the note a few days ago.
He said the neighbour slipped it under his door around 8.30am.
"They must have come early to make sure I was asleep," he said.
"I was rolling on the floor laughing while reading it, I was mortified."
He said he's unaware which neighbour sent it and "really" didn't want to know.
Cunningham took a photo of the letter and posted it to Twitter alongside the caption:
"Not me waking up to a 'have sex quieter' note from my neighbours, Mortified."
Unsurprisingly the post has attracted hundreds of likes, with some users suggesting he buy his neighbours earplugs.
"I would if I knew which neighbour sent it," Cunningham joked.
Others shared their experiences, with one man saying he would hear his neighbour every night her partner stayed over.
"I tried knocking, saying if they could hear me, I could hear them; I eventually put a note through the door saying 'PLEASE put a pillow at your headboard or move your bed," the man wrote, adding, "two days later, found her headboard in my garden".