During the day people prep their nutrition delivery system for the next several hours. The most common way to do this is by eating. As soon as we run low on fuel, we would usually eat again to fuel the next several hours. Unwinding refers to how we transition from rhythm to prepare for sleep at which time our cells eat much more slowly than they do during the day.
The art of unwinding for sleep is to fill your empty tanks on the one hand but to also prevent flooding the cell on the other hand. Suppose you had a soccer game at 4:00. By dinner time your tanks will be low and your cells will continue to eat extra food for the next hours to replenish their batteries, etc. In that example, you could eat a lot for dinner, and still not flood your engine when you go to sleep.
Assuming things are timed correctly, the cell can transition to a very efficient metabolism. It is easier to imagine how this can happen by imagining how it cannot happen. If the system is flooded with food, it is impossible for the cells to slow down for sleep because that would further exacerbate the flooding with food. The cells instead would have to stay active to burn off the fuel or divert the food to fat cells, etc.
But it is not just the timing of the eating that determines the transition. If you are worried about something, your system will get very active as well. The psychological element of unwinding is very obvious, and there is another element that has to do with both the psychological and the nutritional.
Our bodies have two distinct physiologies that oppose each other. During the daytime when we are performing, our cells are always with some degree of acidity. That is true to the degree we have to perform. The harder the cells work, the more they create acid. The more this happens, the more of a priority it is to get rid of acid. What happens is that our brain gets sensitized to carbon dioxide, which translates into a faster breathing pattern. This faster breathing helps us get rid of acid indirectly in the form of carbon dioxide. Presumably, as we relax, our brains would desensitize to carbon dioxide, which would make us breathe slower. It is this slower breathing is required for us to unwind. The primary way the cells know to start taking in oxygen is the presence of carbon dioxide which makes sense considering carbon dioxide is the signal the body is using oxygen. It is the simultaneous relaxation of the mind with a slower breathing pattern without too much food in our system that characterizes the transition to a more intense daytime physiology to a more relaxed and efficient metabolism conducive to sleep.