Personally, I think napping is feature doesn't add very much to the game. Their energy barely increases and sometimes my sims chose to nap instead of actually sleeping and it wastes time.
I only use napping if they are still in the green. It gives them enough to make it thru school/work or until bedtime.
If they are in the yellow, I use sleep (no matter what time of the day or night it is). Then wake them up when I think it will keep them going till bedtime.
Toddlers.....different. I rarely use nap with them. They sleep twice a day in my games. Once around 1 or 2 pm and then around 8 or 9 pm.