+1

I would take the day flight if you can be at JFK early enough. That way you'll really be able to enjoy the flight - the advice is to stay awake for the flight as this helps beat jetlag on arrival.

I did the sleeper service from JFK once and thought it was fine, but despite sleeping from about 30 minutes after take off to breakfast being served, I still was absolutely shattered in the morning as 8am in London is still 3am New York time whether you've had a few hours sleep on board or not!

In terms of crew disappearing, half of the crew will go to their crew rest and half will remain in the cabin, and they swap over half way through.