Red eyes are in part a by-product of the hub and spoke system. Aircraft have to be returned or "positioned" from the west coast to midwest/east coast hubs anyway so the airlines might as well try to make a little $$$ on them while they're at it. Southwest aircraft utilization is already pretty high...at some point aircraft need a little down time for maintenance overnight is the perfect time. Also, SWA does a good bit of overnight charter work that also accounts for some of the fleet allocation. As easy as it might seem to squeeze a couple of extra airframes out the fleet for red-eye flights, it's tough to do when you operate nearly 3,000 daily departures. Every aircraft headed eastbound is one less available for the next morning's west coast originators. Load factors on red-eyes tend to be a little low. Southwest's break even load factor is already low compared to industry standard making the margins on red-eyes even thinner. Compounding the problem on the cost side...I know we Flight Attendants and perhaps pilots too make additional pay for overnight flights. In the end though, if the Customers want it and the revenue stream is there to support it, it will eventually happen. Southwest is still a relatively new player in the transcon market, who knows what will happen in the future.