A 48-year-old man has been sentenced to three-and-a-half years' jail for a hit-and-run crash that left cyclist and Perth university professor Thomas Lyons, with a serious and ongoing brain injury.

Professor Lyons, 66, a climate change specialist, was left lying unconscious on the side of the Esplanade in Mount Pleasant in September 2015 after he was hit by a ute driven by Stephen Francis O'Brien.

O'Brien did not stop and in the days that followed told his wife he did not know how his car had been damaged and then made a botched attempt to repair it.

He handed himself into police four days later.

Last month O'Brien was convicted of charges of dangerous driving occasioning grievous bodily harm, failing to stop and render assistance to Professor Lyons, failing to report the crash and attempting to pervert the course of justice.

Prosecutor David Jubb told the District Court Professor Lyons had been left with "ongoing serious disabilities", including having difficulty swallowing and reading, and his condition is "continuing to deteriorate."

Professor Lyons was hit while riding his bike in Mount Pleasant. ( ABC News )

'Valued community member left crippled'

At his trial, O'Brien claimed he had no memory of the crash because he was suffering from chronic sleep deprivation and should not have been driving.

Judge Philip McCann said he was satisfied O'Brien had fallen asleep at the time of the crash and that he had also been aware his ability to drive was "impaired by tiredness."

Judge McCann said the offences had left "a healthy and valued member of the community crippled."

O'Brien's lawyer Simon Freitag SC said his client had been going through a "low ebb" at the time and he had wanted to apologise to Professor Lyons much earlier on, but had received legal advice not to.

However, Mr Freitag said O'Brien had now written a letter which he hoped would be passed onto Professor Lyons and his family.

Judge McCann said the most serious offence was failing to stop and help Professor Lyons, especially because the crash happened at 6:00am when few people were around.

"It was both his duty and necessity to stop," the judge said.

"He left Professor Lyons at significant risk and he did not even raise the alarm."

Judge McCann accepted that O'Brien had since rehabilitated himself and said he would have considered imposing a suspended term, if not for O'Brien's actions after the crash.

He sentenced him to three-and-a-half years' jail and disqualified his driver's licence for three years.

O'Brien will have to serve 21 months before he can be released on parole.