In order to understand the laws pertaining to Shabbat, we must first understand the basis and underlying nature of the Day of Rest.

The reason we rest on Shabbat is because "It was during the six weekdays that G‑d made the heaven, the earth, the sea and all that is in them, but he rested on Saturday. G‑d therefore blessed the Sabbath day and made it holy (Exodus 20:11)."

Now, we must understand what the Torah means when it says that G‑d "rested." Was He tired? How can an infinite, omnipotent G‑d need to rest?

Besides, the Torah tells us that G‑d created the entire universe by speaking. Did that exhaust G‑d to the point that He needed to unwind?

The answer is very simple. G‑d did not rest from exertion—he rested from creation.

Creation is not necessarily synonymous with exertion and physical effort.

On the seventh day, G‑d rested from creating. On Shabbat, we too abstain from "creating."

What constitutes "creation?"

Our sages have synthesized the paradigms of creation into 39 categories. There are 39 categories of activities considered to be creative. These 39 categories of activity were necessary for the construction of the Tabernacle that G‑d commanded the Jews to build in the desert. All construction was to be suspended on Shabbat, despite the importance of building the Tabernacle. This serves as the source for determining which activities are considered "creation" and therefore prohibited on Shabbat.

Once it is clear to us that on Shabbat we abstain from creating and not necessarily from exertion, things make sense. When we drive a car, for example, we are creating fire (in the internal combustion engine). When we turn on the light, we are creating an electrical circuit. And so on with all other Shabbat prohibitions.

So, it's not an issue of stressful vs. unstressful—it's an issue of creative vs. non-creative.

Also, the six days of the week are days in which we transform the world outside of us. On Shabbat, however, we dedicate ourselves to our internal, personal, and spiritual world. In Kabbalistic terminology, the six days of the week belong to the "World of Speech," whereas Shabbat is the "world of thought." (One speaks in order to communicate with others. One thinks in order to communicate with oneself.)

Therefore, on Shabbat, we retreat from interaction with the tumultuous material world and turn our concentration to our personal spirituality.

Under what category of work prohibited on Shabbat does turning on electric lights fall, and why?

Here’s the short answer: By turning the lights on you are causing a fire right there in your home—there are sparks of fire created by the electricity.

Another school of thought explains as follows: The switch closes a circuit which allows the electricity to flow, and therefore falls into the category of “the final hammer blow” or completing something on Shabbat that was incomplete prior to Shabbat (the circuit).

Yet another reason has to do with cooking: The electricity heats the filament in the bulb to the point of changing it, which is considered 'cooking metal'.

Remember, working on Shabbat does not mean doing strenuous labor. It means doing creative acts that manipulate nature. That's what is not allowed on Shabbat.

When G‑d rested on the seventh day he was not suffering from over-exertion. He rested in the sense that He ceased creating. We must do the same.