Without gravity: 437.7 days.

Admittedly, floating in space looks like great fun, but not for 437.7 days, which is the world record for the longest single spaceflight set by Russian cosmonaut Valeri Polyakov in 1995.

Why you shouldn't try this: The human body is built to function under Earth's gravity. Without it, astronauts' muscles, including those in the heart, lose mass and strength.