Do flight regulations prohibit sleeping on the floor of an airliner? During a recent trip in a Finnair A350, a couple traveling together was occupying 1 row of three Economy Comfort seats. During the 12 hour flight, one of the couple stretched out on the seats, the other one on the floor in between the seat rows, leg room being sufficient to allow this.

A flight attendant was overheard stating that sleeping on the floor was not allowed due to safety regulations. What safety regulation would prohibit this?

update

During cruise condition, no Fasten Seatbelt sign. EASA regulations. Yes you're out of your seat but we're allowed to walk around aren't we. Where is the hard, written, safety regulation?
That depends on the air carrier and the regulatory body they're certified by. Finland is part of the EU so I'm assuming that Finnair's certificate is under EASA aviation safety regs. Specifically, a quick Google search finds:

CAT.OP.MPA.165 Passenger seating:
The operator shall establish procedures to ensure that passengers are seated where, in the event that an emergency evacuation is required, they are able to assist and not hinder evacuation of the aircraft.

Finnair would also write their own SOPs to ensure they meet all the EASA regs.