An air conditioner frequently freezes up overnight due to a combination of lower ambient nighttime temperatures and restricted airflow. When outdoor and indoor temperatures drop during the evening hours, the heat load on your home decreases significantly. If your AC system is slightly oversized or if your indoor air filter is partially clogged, the lower nighttime temperatures cause the evaporator coil to drop below 32 degrees Fahrenheit. As household humidity condenses on the super-cooled metal fins, it turns into a solid layer of frost that accumulates and builds into a block of ice over the course of the night. Verification: Check your indoor evaporator coils for ice buildup each morning and inspect your air filter cleanliness.
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