Get off the expressway and take another route. Keeping yourself occupied with changing gears, braking, stopping, accelerating, slowing down, changing lane etc etc can help to keep you awake.
One of the most common causes of drowsiness while riding is boredom.
I just took a 530km 9 hour ride up to Terengganu via trunk roads and at no point was I sleepy. And I have fallen asleep on just a 2 hour ride up to KL on the NS Highway. It does get very boring if there is nothing much to do, even though the speed may be faster.