Short answers, pulled from the story.
Valeri Polyakov holds the record for the longest single continuous spaceflight in history, spending 437 days, 17 hours, 58 minutes, and 31 seconds aboard the Mir space station during his 1994-1995 mission. His combined time across two flights exceeded 22 months, or just over 678 days.
Polyakov volunteered for the 437-day mission specifically to study how the human body responds to micro-gravity over a duration equivalent to a crewed transit to Mars. He underwent medical assessments before, during, and after the flight, as well as two follow-up examinations six months after returning to Earth.
After returning from his first 240-day mission aboard Mir in 1989, Polyakov's first words were "We can fly to Mars." He then spent years advocating for a second, longer mission to gather further physiological data.
Polyakov broke the previous spaceflight duration record on the 9th of January 1995, after spending 366 days in space on his second mission. The record he surpassed had been set by cosmonauts Vladimir Titov and Musa Manarov roughly six years earlier.
Researchers found no cognitive impairments in Polyakov after his 437-day flight. They did document a clear decline in mood and a sense of increased workload during the first weeks of the mission and during readaptation on Earth, but his mood stabilized to pre-flight levels between the second and fourteenth month. The findings supported the conclusion that a stable mental state can be maintained on long-duration missions.
Polyakov received the Hero of the Soviet Union, the Hero of the Russian Federation, the Order of Lenin, the Order of Parasat from Kazakhstan, and the Prince of Asturias Award from Spain. France recognised him as an Officer of the Legion of Honour.