Window fans have serious limitations. Sometimes at night they can make a room too cold. Other times during the heat of the day, they pump hot air into a room making it too hot. Reversible fans are great but I don't usually remember to flick the switch to exhaust from the room in the morning and the room quickly gets hot. Worse yet, I never remember to go upstairs and turn around the switch when it starts to cool down outside in the evening and the fan just keeps drawing air out of the room.

A possible solution would be to have two temperature sensors, one inside a room and one outside the house. When it's cooler outside than inside, but above a certain minimum temperature, the fan would run. The problem for me is that the outside is on a second floor and it is tough to get a probe outside and keep it weatherproof. I conclude that a solution needs to be inside.

Another approach would be to run the fan when it was dark outside as long as it wasn't too cold. That would turn the fan on around sunset when it's getting cooler and shut the fan off when it started to warm up. Even in a more northern latitude it gets fairly dark by 9 PM or so and if I go to bed around 10 AM, the fan should go on just fine for about an hour or so before bedtime cooling down the room. In the end, this is a pretty simple control algorithm. Run the fan as long as it is dark and above a certain temperature, say 55 degree F.