A court heard that Jeffrey Ash could no longer cope with Ellen Ash's terrifying hallucinations, her failure to recognise her son and the "onerous burden" of looking after her almost single-handedly.
Jailing Ash, 50, at the High Court in Edinburgh, judge Lord Pentland told him: "The primary concern to which I must have regard in selecting the appropriate sentence is the fundamental principle of the sanctity of human life.
"No matter how difficult circumstances had become you had no right - as you now acknowledge - to take her life."
Ash's duty was to seek further support, he added. "You will always have to bear the burden of guilt," the judge said.
Mrs Ash died in the house she shared with her son in Everard Drive, Colston, Glasgow, just a week after she was sent home from hospital.
In the early hours of March 21, a passer-by saw smoke coming from the property and dialled 999. Firefighters made their way into the house and discovered Mrs Ash's severely burned body on the living room floor.
Experts said the fire had been started deliberately and bottles of turps and white spirit were found in the house.
Advocate depute John Scullion, prosecuting, told how Ash fled to London on an overnight bus but, that afternoon, walked into a police station and confessed: "I killed my mum last night. I smothered her, then I burned the house down.
"I could not see her suffering any more. She had Alzheimer's."
First offender Ash had previously pleaded guilty to culpable homicide and wilful fire-raising, although it was accepted no-one else had been put in danger by the fire.
When Ash appeared for sentencing yesterday, Donald Findlay, defence QC, said: "This is a case which can, with absolute justification, be described as a very sad matter indeed."
Ash was still going through "a grieving process", said the lawyer, and he recognised he should be punished.
"He took the life of the person who meant most to him in the world," said Mr Findlay. "He is someone who has expressed huge guilt, huge remorse.
"He felt he should be punished, not for what he has done, but more for his mother's sake. If there were not some form of appropriate punishment, her death would be trivialised."
Mr Findlay asked the court to take into account the pressure on Ash. "In the latter days of his mother's life, Ash was left to do that which no son should have to do."
Ash's sister, living in America, had sent an unsolicited letter supporting her brother, said the lawyer.
The court heard how Mrs Ash had been suffering from increasing dementia and her condition had worsened during the last two years of her life. Her son was her "primary carer" and although he got some help he had repeatedly told health care professionals he could not cope.
Mrs Ash was doubly incontinent, often wandered from home and was frequently confused, but insisted she did not want to stay in a care home.
Ash said he was getting little sleep because his mother was frequently up during the night.
Psychiatrists who examined him after the killing concluded he was suffering from diminished responsibility when he smothered his mother.
Sentencing Ash, Lord Pentland said: "For some time before her death you had shouldered most of the onerous burden of caring for her.
"As a result of the strain placed on you, your mind became increasingly disordered and you began to contemplate killing your mother."
The judge also made an order keeping Ash under supervision for a year after his sentence to help him readjust to everyday life.