Carbon dioxide levels in bedrooms with 2 occupants with the bedroom doors and windows closed can rise to 4,500 ppm during the night…The literature indicates that persons exposed to 10,000 ppm over a period of several weeks experience a measurable bone loss.

Also a paper from a study in the Netherlands called The Healthy Bedroom.

Sleeping in exhaled air
Humans act as emission sources of heat, CO2, and moisture from breathing and the body, bio-effluents. The amount of CO2 produced is related to the food intake and the activity level of each individual person, but is, compared with activity level typical of office worker, about 200 ml/min. Carbon dioxide levels greater than 800 ppm (1000 ppm) indicate poor air quality. The typical CO2 outdoor concentrations vary between 350 to 575 ppm.

The ventilation conditions in 35 bedrooms in family houses in La Coruna city (Spain) show, that CO2 levels are much higher than recommended. This was also found in monitored houses in Rotterdam, The Netherlands. The maximum CO2 concentration in the bedroom varied between 2300 and 5480 ppm (Rodriguez, ). Values higher than 3000 ppm were reached when the bedroom had the door closed, even when there was only one person. The average infiltration rate varied between 0.66 and 5.04 dm3/s in all cases and was lower than the minimum ventilation rate required for appropriate indoor air quality.

If you leave your windows open a crack in a cold winter, you are throwing money out the window – a lot of money. Don’t waste energy.