I've come across once this kind of expression somewhere:

Smith's Law of Inactivity:
Human body, when it is in a repose, tends to watch TV.

Is this meaningful as a joke to you? Or how should I modify it....?

Thank you for your kindness, in advance. I'll be grateful for your opinion.
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This is meaningful as a joke, and I don't think you should modify it. There are a number of these "laws"/ adages, the most famous being parkinson's law
Work expands to fill the time allotted to it

In other words, however long you have to do one job, you will use all the time available to do it! You could do the job in 2 hours, but if you were given 8 hours, then that's how long it would take.

Here are some more of these adages:

Human body, when it is in a repose, tends to watch TV.

I know something about inactivity. Nonetheless, I feel the sentence could be tweaked a little:

'The human body, when (it is) in repose, tends to watch TV.'

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Imagine all your pores or hair roots growing tiny retinae...
Or tympana...The entire body evolving into one vast organ of perception – except for the central ever-grinding orifice, where beer and cheesy snacks would be deposited at regular intervals.

(do they have TV over there? or beer...)
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Limbs would wither away...except for giant right thumbs, to manipulate the remote control.

(But then, who would fetch the beers and cheesy snacks? A sudden flaw...)

Why, you would have a robot, of course! By this time, you would have grown roots and be adherent to the sofa.
.... what a beautiful image ... it reminds me of one passage from "Marbles", a play written by J. Brodsky. Please let me quote:

(Tullius) .... The main thing is time. Thus we were taught by Tiberius. Rome's task is to merge with time. That's life's meaning. To get rid of sentiments. Of all these la-di-das about women, kids, love, hate. To get rid of these ideas about freedom. And then you'll merge with time. For there will be nothing else but time. And after that you needn't even stir -- you'll be going along with it.
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