Another factor of drowsiness that was not included in the DoT study is altitude, and the lack of oxygen at higher elevations can affect your mental state even more at night. If you're flying in a non-pressurized airplane, breathing supplemental oxygen will help. I would suggest O2 above 5,000 feet at night and 10,000 feet during the day. It's a small price to pay for increaed alertness. Make sure you set up and test your equipment before signing yourself off for the flight.