Ron Reagan's book about his father discusses the very strong possibility that he had symptoms of Alzheimer's disease during his presidency:
In his memoir “My Father at 100,” Reagan writes:
“Today we are aware that the psychological and neurological changes associated with Alzheimer’s can be in evidence years, even decades, before identifiable symptoms arise. The question, then, of whether my father suffered from the beginning stages of Alzheimer’s while in office more of less answers itself.”
Ron Reagan recounts having concerns as far back as 1984.
"Watching the first of his two debates with 1984 Democratic presidential nominee Walter Mondale, I began to experience the nausea of a bad dream coming true. At 73, Ronald Reagan would be the oldest president ever reelected...[M]y father now seemed to be giving them legitimate reason for concern. My heart sank as he floundered his way through his responses, fumbling with notes, uncharacteristically lost for words. He looked tired and bewildered."
Two years later the president expressed his own concern about his failing memory:
“My father might himself have suspected that all was not as it should be. As far back as August 1986 he had been alarmed to discover, while flying over the familiar canyons north of Los Angeles, that he could no longer summon their names.” ...
“I have since learned from a doctor who happened to be interning at the hospital when my father was brought in that surgeons involved in his care, in what my informant characterized as ‘shameful’ behavior, violated my father’s right to medical privacy by subsequently gossiping about his condition.”
The question of whether Reagan was all there during his presidency was a major controversy. Rick Hertzberg's masterful 1991 review essay summarized the evidence of Reagan's doddering obtuseness:
Reagan, as portrayed in Caninon's book and in his own is a childlike and some times childish man. His head is full of stories. He is unable to think analytically. He is ignorant. He has notions about the way things work, but he doesn't notice when these notions contradict each other. He has difficulty distinguishing between fantasy and reality. He believes fervently in happy endings. He is passive and fatalistic. He cannot admit error.
Within the White House, Reagan himself was consulted precisely as one consults a horoscope. To his frazzled assistants he had mystical power, but was not quite real. Like a soothsayer's chart, he required deciphering. "Reaganology,' Cannon writes, "was largely based on whatever gleanings could be obtained from body language." The president's pronouncements in meetings, which usually took the form of anecdotes that might or might not be relevant to the matter at hand, were open to various interpretations. When the conversation ranged beyond the handful of Animal Farm-type certainties that made up what Cannon calls Reagan's "core beliefs" (taxes bad, defense good; government bad, markets good) Reagan was lost. Though the people who served with him respected him for his occult powers — his rapport with the television audience his ability to read a text convincingly, the powerful simplicity of the core beliefs — they viewed his intellect with contempt. They thought he was a big baby, and they were right.
Conservatives have made it a point of pride to portray the Gipper as a deeply-engaged intellectual. A 2001 book of Reagan's speeches and commentaries created a huge splash among movement conservatives, who insist that Reagan was "an intellectual force" and "not only an active mind but one more engaged than the critics (and even some friends) imagined."
But, of course, Reagan's ability to craft speeches and prose during the 1970s, and the beginning of his presidency, hardly refutes the notion that he grew progressively more addled as the disease took its toll.