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Flies generally prefer warm weather and sun, so they aren't active at night. Like most people, flies rest when the sun goes down. When the temperature rises during daylight hours, they go back on the hunt for food. If you want to find where your houseflies rest at night, look for tiny dark spots or "fly specks" on flat surfaces -- these mark where resting flies have regurgitated and excreted waste.
Light and Temperature
While they rest at night, flies are active during the day -- particularly when it's sunny and warm. According to the Cornell University Department of Entomology, they get most of their activity when it's sunny and the temperature is between 80 and 90 degrees Fahrenheit. When the temperature dips below 45 degrees Fahrenheit, or when the sun goes down otherwise, they return to their resting state.