A little bit of water collecting inside your car's headlights is perfectly normal, but too much condensation can be hazardous. Moisture forms inside the housing of the lenses when temperatures drop at night. As the headlights cool off, whether from use or warmer daytime temperatures, water droplets will form inside the lens. In the morning, this condensation will evaporate as temperatures rise.
Headlights are made up of a few stacked components. The bulbs are fitted into the housing, and then the reflectors are placed over them. The trim fits onto the reflectors, and then the lens covers everything up and protects the lighting system from the elements.
You might think that your headlight housings are sealed, but this is not the case. There are vents in the lens covers to allow for pressure differences, so water can condense and evaporate inside the headlights. Some housing vents are less effective than others, which can lead to water building up inside the housings. Cracks or holes in the plastic cover can also allow more water into the lens than can evaporate normally.
Regular, expected condensation will form tiny droplets that appear misty, but when excess water builds up in the headlights, it will form larger drops and be far less likely to evaporate. It can also pool at the bottom of your headlights. This can reduce visibility when driving at night, as the droplets can obstruct the light.
If there are small cracks or holes in the lens cover, drivers have a few options to fix the problem. The most obvious answer is to replace the headlight or its affected components, and this may be the only choice drivers have if the damage is severe enough. For minor damage, sealant can be used to plug up the holes and prevent excess water from getting trapped inside the headlights. You may also need to take the system apart and dry it out with an air compressor to get rid of the collected water.
You should also check to see if any components of the headlight are loose. Any exposed space creates another way for water to enter the lighting system. If you find that a light is loose in its casing, you can use sealant to ensure that the connection is airtight. Some headlights are complicated to dismantle, while others are easily accessible. Your vehicle's owner's manual can be useful when you are determining how to take the lights apart if you need to replace any components.