Immortality in The Sims 3 falls into two categories: lengthening your Sim's lifespan, or actually making them younger.
If your Sim has gone to that big gold sparkly swirly bit in the sky, don't worry! You have a lot of ways to handle the situation.
Of course, since the beginning, the Sims franchise has had two exterior lighting effects: daylight and nighttime. And it seems to me like it used to get darker in Sims 2 if there was a thunderstorm (required the Seasons expansion pack). But I may be mis-remembering that.