Is it technically viable to reintroduce sleeping berths? The airlines are always trying to jam more passengers into each plane. I'm smaller than today's average, and I'm still often uncomfortable in a standard Economy seat.

It occurred to me that in the current design, there's a great deal of space wasted over the passengers' heads. And that many of us passengers already do our best to sleep through flights.

Hence my question: Is there any reason an airline couldn't introduce a cabin in which some or all passengers travel in a reclining, rather than sitting, position? Seems to me that  it would be more comfortable (except for claustrophobes and those who really need to work during the flight). The enclosed space would inherently reduce some of the risks of passengers being thrown around in turbulence or an emergency landing; safety belts (or cargo restraint webbing? I'm not sure I'm joking) could handle the remaining risk.

No, I don't really think it would be commercially viable ... but I'm wondering whether folks who actually Know Something about aircraft safety and loading think it's technically viable.
Some airlines already offer sleeping facilities, by taking advantage of the A380's tremendous size:


A double-bed suite on Singapore Airlines




A first-class seat on Emirates




A first-class seat on Qantas


That top bed looks very tempting, but I dread to think how much it costs!