I use to work in Angola on what is known in the oil industry as a 28 & 28 rotational job. I lived in the US and would fly to Luanda, Angola from Houston, work 28 days and then fly home and was off for 28 days. Did it for 5-1/2 years, so about 40 trips back and forth over that time period. The flight was a charter, non-stop, business class flight that was around 13 hours. Would depart Houston around noon time and arrive Luanda around 7 or 8 am the next morning
As several people have mentioned, as soon as you take off, set your watch to the arrival location time zone. I would also get up extra early the day of my departure. I'd get up at 3am, which would be 9am Angola time. This helped to transition to West Africa Time. It also made me very tired once the flight took off and I slept better during the flight. Ear plugs and eye shades help too. Get a seat where people don't have to climb over to get up. I always perferred the center aisle in the wide body jets, with a 2-2-2 seat configuration.
On a safety note, flying these long flights can be hazardous to your health. DVT (Deep Vein Thrombosis), sometimes referred to as "Economy Class Syndrome" is one of the more serious things that can happen. Basically a bloot clot in your leg from inactivity. Get up and move around during the flight. If you have ever noticed that your feet swell during long flights, you might consider wearing compression socks. I had DVT once and it gounded me for about 3 months. Not fun and potentially life threatening.