SoulSmilen, I think there are two issues. One is degrees of weak & I think we've worked that out to be a lack of food - whether or not you feed them with a prepared meal, or just have a well stocked fridge. If your food levels are really low (like I did accidentally the first night of playing), it seems to make them weak.

Lack of rest - sleep, naps seems to cause the tired & exhausted.

At least that's what I've found. Since I've stocked up on food I've not had weak at all, but I still get tired, but a lot less with the 2nd gen family. Not sure if it's because I did reverse the day/night with the 1st gen and maybe they skipped a whole night of sleep!

I am still having issues with sleep with two families running. When I put them to sleep in one game slot, then switch to the other, they automatically get out of bed and wander around getting really tired. Again, not sure if this is because it's a new family 1st generation and it will work out in a day or two.