These days we are too much cooped up in the buildings as in offices and all. So, its good that we get well-ventilated houses. One of my friend's mom used to open the windows even if he used to feel cold at night :D Yup, she was paranoid about his health. Not in bad sense AT ALL.
The reason she was so concerned was because if you keep the room closed for long time, your exhaled carbon dioxide gets filled in the room reducing the levels of oxygen. Same is the case when you sleep with your face covered. It interferes with the normal breathing process.
Another issue which might arise is the entrance of outside light when you open the windows. Yes, one should sleep in extreme dark because our body is sensitive to even a flicker of light when sleeping. So, its a good idea to get a sleeping mask on!!
Did you know that if you do not sleep in an entirely dark room, your ageing process gets accelerated and also results in weight gain. Vivi has already done a wonderful post on this topic which I have been meaning to post since a very long time so go and check it our right away :)
How to get undisturbed beauty sleep of 7-10 hours?
How long do you sleep?
Sleeping posture decides height
Why should you take proper sleep?