The principle is that keeping the lid down helps to keep anything unhealthy in the toilet bowl from circulating freely in the the air. This is important in the bathroom itself, but becomes especially important depending upon where the bathroom is located.
These days, almost all master bedrooms in homes are designed to include a bathroom within the master suite, a design traditional feng shui would not have encouraged; rest and personal hygiene areas were kept separate. Most bathrooms have doors, but they’re generally left open because privacy and appearance aren’t the issue in a bedroom that a bathroom elsewhere in the house can create. This air from a bathroom is then freely flowing through the sleeping area.
There are two ways to consider the effect of this. The first is the physical world which includes bacteria, and possibly any cleaning agents used in the toilet. Every time a toilet is flushed, and even when the water is still, the bacteria and any cleaning product chemicals can circulate through the bathroom and into the bedroom.
The second way is energetic. My first feng shui teacher said that the “dirty energy” we’re eliminating, or that can result from any stagnant water and stagnant air combination elsewhere in the bathroom where oxygen is low, can also promote illness. When the lid is up and a bathroom door is open, this energy also travels out along a pathway from room to room. We did a comparison of simply sensing the difference with our hands when a toilet lid was up or down, by gently palpating the air over the toilet. When the lid was up the air felt denser and a bit uncomfortable, and with the lid down the air was lighter and had a more pleasant quality (done over an apparently clean toilet).
I was taught that if there is “dirty energy” in a bathroom from the toilet or anything else in there that isn’t clean, and the bathroom door is left open, there’s the possibility that someone sleeping in an adjacent bedroom in the path of the air currents or this traveling energy pathway might experience health issues in that part of the body that is lying in the energy pathway during sleep. I had a client who had a cat box in the bathtub to keep the cat litter neater, and the open bathroom door was directly in line with where the middle of the client’s body would be on the bed at night. I asked the client if they experienced digestive issues, and was told they did. The condition subsided when the cat box was removed and the bathroom door left mostly closed.
To keep the air and energy healthy in a bedroom or any room near a bathroom:
Civilizations rise or fall in part depending upon the state of our hygiene, the state of our water and air. Every simple measure we take and habit we have that helps to ensure the safety of water and air directly affects our health and well-being, A simple flick of the wrist to close a lid or a bathroom door can have more cumulative benefit than one might realize.