"Terrible" :S :(
Thats why people should understand you cant biologically drive continously for so many hours, it gets tiring!
Dont be over-confident, if you begin to feel even a bit tired, pull over and have someone else drive for you. Otherwise, pull over or take the nearest exit and studies say 15 minutes of rest should suffice.
But those 15 minutes become extremely important - Some tips to drive when you are getting tired and you HAVE to keep going, :
-Turn on the A/C high speed, cool air will not allow you to rest
-Turn on the radio or loud music ...
-Turn on interior cabin light(s)
-Open the window (slightly) - the noise and wind pressure/temperatures will be against your sleeping.
Take care; drive safe!