Condensation happens when the outside of the lens is cooler than the inside of the lens. When you drive for a few hours, the bulbs warm the air inside the headlight. The same thing happens when the headlights sit out in the sun all day. 

Once the air inside the headlights is warm. If you turn your lights off or pull your vehicle into the shade, there will be a large temperature difference between outside and inside the headlight. The cool air wants to get inside and it begins to sneak into the tiny vents that are found on most headlights. When that cool air meets with the warm air, condensation is formed. That happens on the warm side of the lens, so you have got a little layer of moisture on the inside of the headlight lens.

Virtually all lights are prone to condensate due to the following events: Air is trapped inside a lamp when manufactured Plastic parts used in lights naturally absorb moisture in the air.  As the light heats up these plastics parts release moisture that mixes with air. Moisture attaches itself to the coolest part of the lamp, e.g. the lens. Halogen lamps tend to run hot so the moisture evaporates more quickly. LED lights produce less heat so moisture takes longer to evaporate, so it can be more noticeable.