This reportedly from a Prussian spy who visited the Marxes in the 1850s:

In private life he is an extremely disorderly, cynical human being, and a bad host. He leads the existence of a real bohemian intellectual. Washing, grooming and changing his linen are things he does rarely, and he likes to get drunk. Though he is often idle for days on end, he will work day and night with tireless endurance when he has a great deal of work to do. He has no fixed times for going to sleep and waking up. He often stays up all night, and then lies down fully clothed on the sofa at midday and sleeps till evening, untroubled by the comings and goings of the whole world.

In a world without Friedrich Engels that last sentence might have ended with, “except when he dreams has nightmares about the job market (or Reviewer Number 2).”