Descartes would never get out of bed before 11 a.m. No, he wasn't lazy—it was a habit that went back to his childhood, when he was constantly sick. (Source.)
Once he moved to Sweden, Descartes changed his routine and began to get up at 5 a.m. What else could he do? It was the request of the Queen herself, who liked to start her mathematics lessons bright and early. Big mistake, though: the early rising time, combined with the cold of Sweden, brought on a bad case of pneumonia... and that was the sad end of our buddy René, at the ripe old age of 53. (Source.)
A famous person's death is hardly complete if there isn't at least one conspiracy theory surrounding it. And sure enough, not everyone buys the standard story that Descartes died of pneumonia. That's way too lame for the Father of Modern Philosophy... right? Well, some people have suggested that Descartes was poisoned—by a Catholic priest, no less. Why? Because his philosophical theory was not consistent with the Catholic view of transubstantiation, of course. (Source.)
When he was young, Descartes got in a duel with another gentleman—over a lady, of course. Descartes won. (No wonder he has a reputation as a great "dualist"— har har.) He actually spared his defeated rival. (Source.)
Descartes fathered an illegitimate daughter named Francine with his servant Helena Jans van der Strom. Descartes always referred to Francine as his niece, but she did come to visit him often, and he planned to pay for her education. Tragically, she died at age five. (Source.)
Descartes had a seven-year affair (of sorts) with Elisabeth of the Palatinate (a.k.a. Princess Elisabeth of Bohemia). He even dedicated his Principles of Philosophy to her. But don't get the wrong idea—Descartes assured everyone it wasn't that kind of affair; it was just a high-minded philosophical correspondence. Whatever you say, D. (Source.)
There is a rumor that Descartes was a die-hard Black-Eyed Peas fan and originally wrote the cogito as "I think, therefore will.i.am." But Shmoop can confirm that this rumor is totally false.