Sales consultant James Moran (60) was also over the legal alcohol limit when his Mercedes Vito van brought chaos to the A876 road near the Clackmannanshire Bridge.
He had driven through the night from Manchester after drinks at a charity golf event.
Sauchie father-of-two Peter Harper (44) died of multiple injuries, trapped in the wreckage of his silver Vauxhall Astra.
Jailing Moran at the High Court in Edinburgh, Judge Lord Turnbull told him the prison term was not meant to measure the Harpers' loss. He added, "It will probably seem insignificant in comparison with the sentence you have imposed on that family."
Moran of Cloves, Menstrie, pleaded guilty to causing death by dangerous driving on 5 August last year.
The court heard that a firefighter and a passing GP who were at the scene were unable to save Mr Harper, who had been driving to work in Broxburn as a site manager with Marshall Construction.
Advocate depute Kath Harper, prosecuting, told the High Court in Edinburgh that Moran - also a father-of-two - had left his Menstrie home about 6am on 4 August 2011 to take part in a charity golf event. His friend, James Marshall, accompanied him during the four hour journey.
The two finished a day of golf about 6pm then went to a dinner and presentation at a hotel, where wine was served.
Ms Harper said Mr Marshall stayed in the bar and, when he went to the room he was sharing with Moran, the other man was already in bed.
Soon afterwards Moran woke Mr Marshall and asked if he wanted to go home. The court heard that Mr Marshall slept most of the way and Moran dropped him off at his home on Armadale, West Lothian.
Moran then started out on the final leg of his journey to Menstrie. After crossing the Clackmannanshire Bridge he swerved, forcing another car onto the grass verge to avoid a collision.
Driver John Copeland (43) later told police Moran had his chin on his chest and his head was moving with the motion of the van. Mr Copeland drove on, watching in his rear-view mirror as Moran's Mercedes Vito strayed into the south-bound lane, into the path of three on-coming vehicles.
He hit a grey MG causing the car to spin then collided with Mr Harper's Vauxhall. The force of the impact lifted both vehicles from the ground.
Behind them was a blue Kia.
Ms Harper said, "The deceased's silver Vauxhall car turned clockwise in the air and landed across the top of the Kia's bonnet. The Vauxhall continued to move through the air and came to rest on top of the crash barrier."
A passing firefighter and GP found Mr Harper trapped and slumped unconscious over his steering wheel. Moran was taken to Forth Valley Royal Hospital where blood samples confirmed he was just over the legal limit.
He said he could not remember what had happened, but accepted he had fallen asleep at the wheel, said defence advocate Barry Smith.
Moran said he did not know how much he had drunk, because people kept topping up his glass, but he would not have driven if he had felt unfit.
Mr Smith said Moran apologised to the Harper family. "He has a sense of all the damage this has caused and will continue to cause. He sincerely regrets his action. He will struggle to come to terms with what he considers to be a gross error of judgment. He is tormented that a man died as a result of his action when he himself survived."
Moran - who has four speeding offences on his licence - was also banned from driving for 10 years.