Feb 2020, phys.org
"Romantic Partner" they call it.
155 people, sweaty t-shirts, you get the idea.
You don't even have to be there, just your shirt: "effect similar to taking oral melatonin supplements."
I'm going to chime in here and say this is something to do with security and familiarity. Knowing that another member of your tribe is with you while you sleep makes you less paranoid of the silent creepers waiting to attack you in the dead of night, be they lions, tigers or viruses. And your body knows who is a member of your tribe via both olfaction and your immune system (which are inter-related by their chemosensing inputs).
Your immune system knows who's deep inside your network because their microbes are deep inside your body crawling and growing and metabolizing all over your face, in your nostrils, under your fingernails, and throughout the entire length of your digestive system. Your immune system has one job, and that's to keep the xenobiotics at bay.
Yes, your immune system is hyper xenophobic. As you move through life, you pick up all kinds of microbes from your environment and especially from the people around you. All the while your immune system is working like crazy to detect, attack and remove all those microbiological trespassers.
If you spend enough time with certain people, then your immune system will need a way to identify those people and their biomes so that it doesn't waste time attacking things that aren't actually a threat. It's a probability game -- if you hang out with these people a lot, then they probably aren't a threat, and neither are their microbodies; you'd be better off saving your energy for another time, like when you're at the gym swimming in a sweaty soup of other people's effluence.
Point being, the more immunologically exposed you are to another person, the more your immune system will "know" who they are by the way they smell, and even while you sleep. And when your immune system is relaxed, so are you. (And yes, immunological exposure means exactly what it sounds like.)
Image source: Dartmouth College Electron Microscope Facility
Stanford Medicine, Sep 2018
For two years straight, Michael Snyder, MD, professor and chair of genetics at Stanford, sported a peculiar accessory — a little gray box strapped to his bicep taking sips of air and recording his exposome cloud.It turns out, at any given time, we are bombarded by a combination of microbes, fungi, chemicals, viruses, particulates and even tiny microscopic animals, a new paper in Cell reported. This whirling plume of particulates is called the human exposome.The long-term goal, Snyder said, is to simplify the device into something that resembles an exposome-monitoring smart watch that can suck up and analyze the atmosphere on its own.