Hi Sassy. My first gen family was exhausted all the time, and the papa was sick all the time as well. I was pretty low on funds at that time so I couldn't send all of them to the doctor and I'm not sure what was causing it. My suggestions may not be worth much but here they are:
Leave the game alone for 6-8 hours at night, whenever that is depending on your switch day/night setting. Don't switch the setting back and forth; pick a play time and stick to it.
Make sure they have plenty of food in the house. When you start playing after the rest period, have someone make a meal and make every family member sit down and eat until they are full. When they get up, I sit them down again to top them off, just like my car at the gas station!
Make sure you have enough beds for all your peeps. Sure, they can nap and even sleep on sofas, hammocks, and the like, but my impression is that they get better rest when there are enough beds.
Use fresh fruit when it is in the Flea Market. I have no idea if this makes a difference, but it sounds good. Other things like this are make sure they're clean (feeling fresh), happy (watch DVDs and read stories), let them exercise and give them a green glove for it when they do, and don't let them work all the time.
Here's a little spoiler hint about feeding your peeps:
It takes much less time to prepare a picnic than to prepare a meal, if you have the picnic table. I placed it on the deck outside the kitchen/office area. One drawback is that if the weather is bad - meaning anything but sunny - then the peeps will not want to have a picnic.