"We think he is OK," said Barbara Mayall, who has been at her 39-year-old husband's bedside in a hospital intensive care unit since he was flown unconscious to hospital after the accident in south Devon on Thursday evening.
She told journalists at Derriford Hospital, Plymouth, where her husband is being treated: "I am looking more optimistic."
Mr Mayall has been under sedation in the hospital's neurological ward since he was admitted. A hospital spokeswoman said that his condition was still stable, but was "looking more hopeful", and added that he spent "a comfortable night".
"He is almost through the critical initial 72-hour period of waiting for the brain to stop swelling," she said.
The family has not given any specific details of Mr Mayall's injuries or treatment.
Two of Mr Mayall's Comic Strip comedy partners, Peter Richardson and Adrian Edmondson, visited him in the hospital yesterday, said the spokeswoman.
Fans of the comedian have been telephoning with good wishes - one called from Australia this morning - which have been passed to the family.
Several bouquets of flowers for the actor have also been delivered to the hospital.
The accident happened when the star was using the four-wheeled machine at Pasture Farm, near east Allington in the South Hams.
His country home is a large stone and whitewashed farmhouse set in its own valley well off the road.
It is understood that the quad bike toppled on top of Mr Mayall as he rode across a steeply sloping field.
His wife went out to see why the engine had stopped, and discovered him trapped underneath and unconscious.
The ambulance service called in the Devon and Cornwall Police helicopter to fly him to Derriford, which has the region's specialist neurological ward, because of concern about his injuries.
Mr Mayall bought the country estate last year as a retreat for himself, Barbara, his wife of 12 years, and their three children Rosie, 11, Sidney, nine, and two-year-old Bonnie.
The actor received his first TV break appearing as comic Brummie Kevin Turvey on A Kick Up The Eighties, but shot to fame in The Young Ones.
He also starred in The Comic Strip Presents, Bottom and the New Statesman.
Mr Mayall appears tonight with other Comic Strip stars in a Channel 4 comedy special Four Men in a Car, which is going ahead at 9.30 pm after the programme makers consulted his family.