Sheila Munro, a whitewater kayak and ski instructor, nodded off while her four-year-old granddaughter was with her.
Their Land Rover drifted across the A9 and Munro awoke to find herself smashing into a Nissan being driven by Ruth McLeod. Munro's vehicle flipped and both she and Miss McLeod suffered serious injury, although the girl was unhurt.
At Perth Sheriff Court, Munro, 60, of Swanston Grove, Edinburgh, admitted driving carelessly and falling asleep before causing the crash on the A9 near Bankfoot, Perthshire, on 15 April this year.
Fiscal depute Stuart Richardson said Miss McLeod suffered a fractured eye socket, broken finger and bruising, while Munro fractured her sternum.
Mr Richardson said: "The police spoke to her. She said she was tired and closed her eyes momentarily and when she opened them she was heading to the wrong side of the road and couldn't correct the vehicle."
Solicitor Paul Ralph, defending, said: "She realised that she was starting to feel tired. She decided to come off the road at Stanley. She was half a mile or so from pulling over, when suddenly this accident occurred."
Sheriff Richard MacFarlane told Munro: "Tiredness can hit when you are least expecting it.
"I was not given any reason why you started to feel drowsy, or whether you had been over-active in the pursuits you indulge in."
He banned Munro for four months and fined her 300.