I wouldn't turn down the opportunity to upgrade to F in this instance, but only because I sleep better in F. Perhaps if I got one of the two middle seats and there was a stranger in the other middle seat I might hesitate and stay in CW.

However, if, like the OP, I found it easier to sleep in CW, I'd refuse the op-up because on overnight flights a comfortable sleep is everything and nothing else matters.

I guess people get bewildered because they assume that everyone is the same - i.e. everyone will take an upgrade because they perceive it to be better than what they already have. However, it is the true mark of the frequent flyer to refuse an upgrade where the comfort level is a step down in the higher cabin despite all the extras that come with it.

I once rented a mid-sized car and got upgraded to an S Class. While it was fun to drive on the faster roads, it was actually far too big to fit in a standard sized car parking space and far too big to navigate around the airport car park (I scratched it on the way out but did not get charged for it). A great lesson not to jump at the chance of an upgrade (at the car rental desk, on the plane or anywhere for that matter) until I judge it against what I already have