Christopher Thornton, 36, from Bridge of Allan, attached a picture to the message to his partner showing a pack of four caffeine drinks, a court heard.
His car struck a bike carrying Malcolm Traquair and Ruth Keighley, of Bearsden, on the A9 near Kingussie.
Ms Keighley spent 16 months in hospital after the crash on 23 August 2013.
The 63-year-old cardiac nurse suffered brain damage and had to learn to walk and talk again after several surgical procedures.
Mr Traquair, 62, a former senior fire officer, lost the use of his right arm.
Thornton had been returning home from a business trip to Elgin, the court heard.
In his text, the company director said: "Just leaving Elgin, hopeful of staying awake".
He admitted on indictment at Inverness Sheriff Court to driving dangerously knowing he was tired and causing the couple serious injury by colliding with their bike.
Witnesses described seeing his car drifting towards the wrong side of the carriageway.
Sentence was deferred for a background report until 5 May.
A man sent a text message about hoping to stay awake on his drive home two hours before colliding with a motorbike and seriously injuring its two riders.