This is a picture from when my car was still only less than 1 year old on a cold Autumn morning (Sorry pic isn't the best). As you can see the headlights have 'fogged' up, this is just condensation from the air already in the headlight and is nothing to be concerned about, unless it has a high salt content (Coastal).
If it's happening when the temperature is high then that is reason for concern and a little odd or if it's something physically on the lens (paint overspray etc) you should be able to clean off with a solvent (watch for damage to plastic parts). Or you could try 'drying' the air out, on a hot day removed the bulbs for a bit so the moisture evaporates.