My mum and dad ALWAYS sleep with their window "ajar" as mammy calls it. So even here, when they visit in winter and its as cold as -25 nighttime, they want their window open!!! That's madness....
I usually open all day (or if we're around, all day) in summer and definately leave their windows open till I go to sleep so 7-11pm every evening in all seasons except lower than +5 lets say, if they're well-covered....and in winter, I also open just for a while. I try to keep the heating to the lowest all year round and our flat is very well insulated so our heating costs less than it would in ireland for sure....
In summer, I usually open the terrace on the roof all night and leave all our room doors open - stays darker and quieter that way (we live on a main junction with trams from 4.30am) and with a very chronic EWer, you can't help to think that perhaps, just perhaps, the closed window will give you a few more minutes shut-eye. (Ozzie's a 6am waker and previous 4.30am waker at his worst).
No-one would manage to get our kids - roof is our access only....and we're 4/5 flights up - 5th floor where we sleep is just ours...
S x