Don't keep driving – pull over immediately in a safe place (as far off the road as possible)
Move to the passenger seat and take a 15–20 minute power nap. A short sleep is very refreshing – try not to nap for a period longer than 40 minutes otherwise you may feel groggy and disoriented when you wake up and for quite some time afterwards (this is called sleep inertia)
Wait at least 10 minutes to make sure that you are completely awake before you start driving again.
Note: Stop to revive, then drive: During long trips, take rest breaks about every two hours or every 100 kilometres. If you still feel sleepy don't drive, find a place to sleep for an extended period or for the night.