This is usually true, but last night it didn't really cool down. Or not enough. It was apparently the hottest night on record in the UK.
So yes we did have windows open both sides but there wasn't much breeze and it was warm outside so it didn't really help enough.
Plus we only have one decent fan and we heroically put it in the boy's room as it was far too hot in there and I doubt he'd have slept otherwise.