I travel a lot on planes.  And I sleep a lot on planes.  Or, alternatively, I spend a lot of time on planes feeling tired.  Feeling sleepy on planes is a common phenomenon. What causes it:

1. First and foremost, commercial airliners are pressurized to the equivalent of  8,000 feet.  That is over a mile high – it’s like being in Vail.  Higher altitude means less oxygen in the bloodstream. This causes dizziness, lack of alertness and sleepiness. The reason planes aren’t pressurized to sea level equivalent is for the structural integrity of the aircraft (i.e. keeping the pressure differential between outside and inside within the aircraft’s structural limits). One benefit of the new 787 Dreamliner is that its carbon construction means that it can operate at a higher air pressure (6,000 ft equivalent).
2. The air is dry which leads to dehydration which leads to listlessness. So, it’s a good idea to stay hydrated! Note that coffee and alcohol consumption on the plane adds to dehydration.
3. The ambiance – the lights are down, you are in a partially reclined seat (at least after you take off).
4. The vibration and noise of the engines can lead to sleepiness as does the rocking motion of the plane.

Side note: the higher altitude and drier air tends to dull your senses of taste and smell leading to a craving for more intense flavors. That is why it is more common for people to order tomato juice (or bloody mary) on a plane than they do on the ground. Dehydration also leads to wanting salty foods, like pretzels and peanuts.