This one is not too hard for native English speakers, but we cannot always be sure. The expression is similar to the old-fashioned English idiom, “to take or get 40 winks”. He just felt like taking a nap. I wonder why Czechs need only 20 winks when we need 40. Maybe they work harder. Perhaps, but if that were true, then why do we only have time now for our “power naps”?