Plus, the furnace will pump out even more heat until the window is closed or the thermostat is turned down. If you have a furnace in your apartment, turn down the thermostat when it gets too warm. If your building is heated with a central boiler, you could close the radiator valves in the rooms that are too warm. You could also notify the building manager that your apartment is too warm. The manager may be able to adjust the boiler controls to lower your apartment temperature.
People generate heat.
If you have a group of people in your apartment, let the heat they give off help keep your apartment warm. Turn down your thermostat and save some money. Turn the thermostat back up when they leave. On sunny days, take advantage of the free heat. Open blinds, shades, and curtains, especially if your windows face south, to help keep your apartment warm.