Just an FYI, Warsaw to Paris is in the works. The new EU rules for train operators will allow private operators to operate cross borders so in the future a sleeper train from Paris to Warsaw is going to be a thing. 12h, sleep 8, wait 4. Compared to flying and showing up 3 hours in advance with added to/from airport as 3h pre flight, 2h flight, 1h to/from airport to get to city center you'll have to spend more time waiting in flights with less comfort.