According to the National Sleep Foundation:

Fact #2:  The higher the altitude, the greater the sleep disruption.  Generally, sleep disturbance becomes greater at altitudes of 13,200 feet or more.  The disturbance is thought to be caused by diminished oxygen levels and accompanying changes in respiration.  Most people adjust to new altitudes in approximately two to three weeks.  

Quit Counting Sheep…Sleep with a Wolf!