We do about 50% of the time in winter and 90% of the time in summer. I find spring and fall harder because the temperature is so unpredictable but do open it depending on the weather.
I think that sleeping in a closed room for 12h with the bedroom door shut and no ventilation is unreasonable for my DS - in the morning you walk into his room on the nights when the window has been closed and the whole room is just stuffy and stinky. The heater is below the window, so the fact that the window is open means that the heating kicks in (ie in the winter) and I keep the heat on very low to try to balance out the waste of energy. In the summer the heat is obviously off so not an issue but in the spring and fall it is harder because we can have massive temperature swings either way! Sometimes I open it after he has gone to sleep and close it when I go to bed or sometimes I just open it when I go to bed. Other nights I might peek in on him if I wake to go to the bathroom and if it is hot/cold open/close the window as needed. I used to worry that the extra noise in the mornings, especially in summer, with birds, traffic etc would wake him early but if he is too hot then he will wake anyway so we don't really find any issues with it.
