In comparison to what and how? It really doesn't matter how fast you replace the air. If you let in the cold air then you need to warm it up. What you actuall

In comparison to what and how? It really doesn't matter how fast you replace the air. If you let in the cold air then you need to warm it up. What you actually do is a compromise between energy conservation and properly ventilated room as for the morning the room will be filled with high concentration of CO2 and humidity (from your breathing and sweating) and you more than often end up with bad sleep and headache. Constant proper mechanical ventilation is the key for better sleep and health when windows are goods as German windows (some other European countries make good windows too).