Very brief stop in London

I took a flight from JFK to London Heathrow.  My flight took off at 7am and arrived in London around 7pm.  This was the first time I'd taken a same day European flight - and it was so different from the overnight ones.  I was exhausted and slept quite a bit on the flight because I never went to bed the night before.  This can really screw you up (being exhuasted and going into different time zones).  I did not have a lot of time to spend in London right off.  I was able to squeeze in a late night dinner with a friend and a brief nap before I had to take a cab to Stansted airport to fly to Italy around 6am.