During sleep we pollute the air of the bedroom. That is normal. Ventilation 'washes away' the pollution with outdoor air. In case the ventilation level is too low, pollution of the bedroom air will increase. After some time nose and lungs may become irritated. The sleeper awakes breathless, or continues to sleep but feels tired when awake in the morning.
Sensitivity to air pollution differs among persons. If you have, or ever had, asthma, chronic bronchitis or lung emphysema, you are very sensitive to polluted indoor air.
Clean indoor air is commonly asociated with open windows. Nowadays many persons sleep, however, with the windows closed to conserve energy.
Below we calculate for you how long you may sleep healthy without opening the windows.