A lawyer for a woman found guilty of a fatal hit and run in Victor Harbor has told the Court of Criminal Appeal that she may have been asleep at the time of impact.

Samantha Farrer was found guilty in May of causing death by dangerous driving and leaving an accident scene after she hit and killed cyclist Kathleen Heraghty on Inman Valley Road, south of Adelaide, in 2013.

The District Court previously heard Farrer was driving under the influence of methylamphetamine with little or no sleep after attending a Christmas party and that she had told a passenger she had hit a sign or a fruit stall.

Farrer's lawyer Scott Henchliffe said there was not enough evidence to exclude the possibility that Farrer fell asleep when she hit the cyclist, without knowing she was at risk of falling asleep.

This could lead to a lesser charge of driving without due care.

Farrer driving on 'less than four hours' sleep'

The court heard Farrer could have had less than four hours' sleep before she drove from Collinswood in the city to Cape Jervis on Fleurieu Peninsula to take her friend fishing.

It heard she was driving at a moderate speed and there were no signs of attempts to break before or after hitting the cyclist at 10:40am on December 27.

The court heard police pulled her over later in the day for a cracked windscreen and she did not appear tired when interviewed because she was affected by methylamphetamine.

Mr Henchliffe said the trial judge erred in his conclusions about whether Farrer had fallen asleep while driving at the time of the collision.

"We don't accept His Honour's findings that she didn't fall asleep," he said.

"His Honour rejected [the friend's] evidence, or didn't accept it, on the issue of whether Ms Farrer had had any sleep the night before."

Mr Henchliffe also told the court there was not enough evidence to prove whether Farrer had consumed methylamphetamine before or after she hit the cyclist.

"Whilst His Honour might have made a finding that was more likely Ms Farrer had consumed something before the collision ... there just wasn't enough evidence," he said.

He argued that driving back to Adelaide would have taken more than an hour and this would have given Farrer motive to take methylamphetamine because she did not want to fall asleep.

Farrer aware she was fatigued, prosecutor says

Prosecutor Kos Lesses told the court Farrer was aware she was fatigued and it was highly unlikely she took drugs after the collision.

"Ms Farrer still drove a considerable distance, mostly at night time, and as a matter of common sense it would have been tiring for her as it would any other person," he said.

"The comments 'I was trying to stay awake' shows she knew she was struggling to stay awake.

"On that admission alone she was on notice. She had the tell-tale signs."

The court heard the collision happened on a straight stretch of road with good visibility in perfect weather conditions.

It also heard the deceased was wearing bright, coloured clothing and had a red flashing light attached to her bike.

The court will deliver its ruling at a later date.