The plants function best in moderate temperatures - between 60 and 85 degrees. Temperatures below 50 degrees slow growth in most varieties of marijuana. When the temperature drops below 40 degrees, the plants may experience some damage and require about 24 hours to resume growth. Low nighttime temperatures may delay or prevent bud maturation. Some equatorial varieties stop growth after a few 40-degree nights.

A fan is not needed at night but does have a use, stronger stems. Some growers turn their fans off at night to allow the stems to heal but I know some growers who leave their fans on 24/7 without any problems. If your temperature does not drop below 64 or so, I wouldn't worry about it. Marijuana enjoys a 15-degree drop in temperature at night, according to some articles I have read.

A heater may be in your future if temperatures are an issue now.