Personally I find that if I don't hang out with people in real life every 2-4 days I will get increasingly lethargic and incapable of getting anything done. To what degree this generalizes is another matter.
I find the same thing as Kaj. I've started literally percieving myself as having that set of "needs" bars in the Sims. Bladder bar gets empty, and I need to use the toilet or I'll be uncomfortable. Sleep bar gets low, and I'll be tired until I get enough. Social bar (face to face time) gets low, and I'll feel bleah until I get some face to face time.
The good news is that I've noticed this, become able to distinguish between "not enough facetime Bleah" and other types of Bleah, and then make sure to get face-to-face time when I need it.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
If continuing the discussion becomes impractical, that means you win at open threads; a celebratory top-level post on the topic is traditional.