Long distance driving-sleep problems

Sometimes,  I drive solo, long distances for 8-10 hours. I get drowsy sometimes,  and stop driving. 

However,  I  observed no drowsiness,  if I don't eat on the day of driving.  And vice versa. 

Taking water also helped to stop vehicle after two to three hours of driving and relieve the pressure.  This also made me to have rest for few minutes and resume driving. 

Another observation is to have windows opened instead of using air conditioner,  and fresh air is also relaxing me.