Hola Madrid Experts, I'd really appreciate your advice.
When I leave Madrid I'm catching the early 6:00AM flight to Amsterdam. Rather than staying at a hotel and calling for a cab at 3:00 in the morning, I was thinking of overnighting at Barajas. My plan is to take the Metro to Barajas the previous day to leave my luggage at the 24 hour storage sight and then return to Madrid. I'll then stay out till the last Metro in which I'll return to Barajas. I think I'd be there at 1:00 and wait it out till check in which would be about 4:00AM. I'd retrieve my luggage just before check in. Does this sound like a good idea or should I just stay in a hostal and take a cab at 3:00AM or so? I don't care if I sleep or not: I'm just concerned about safety and comfort. By the way, I'll be travelling solo. Thanks.

A major weakness I see in your plan is something over which you have no control: being able to retrieve your luggage at 04:00 or 04:30. Before you leave your luggage at a consignment area, be sure it is staffed 24-hours. Tell them what you plan to do!

Even if it is supposed to be staffed 24-hours, the employee might be away from his/her post (catching some zees) or even shooting the bull with the taxistas waiting for the early AM International arrivals at the time you want to pick up your luggage and check in for your flight to AMS.

I, personally wouldn't attempt it

Eddie, thanks for the reply. I am also concerned that the "24 hour" luggage storage area may be unattended between 2:00-4:00 in the morning. I checked the Madrid airport site and it does say there is 24 hour service but I'm not sure this is reality. Has anyone here had first-hand experience with the Madrid luggage storage area? Is it too risky to trust that someone will be there during the wee hours? Thanks so much.

The last metro train leaves for the airport around 01:30 AM.

I wouldn't worry about safety in the airport, just about being uncomfortable sitting on a bench all night but if you don't mind...

Tomorrow I'll be going to the airport, I´ll try to find out if the luggage storage area is open 24 hours.

Nevertheless, provided you could leave your baggage on the airport during the day, my suggested option would be spending the night out in the city, and at 3:45 or more taking a taxi to the airport.

Depending on the day you're fliyng there are different options, other than the obvious ones, (bars, discos,...) if you are there on friday of saturday night, late cinema would have original version movies starting as late as 00:45 for example....the risk is oversleeping the movie, seriously if you can place the luggage spend the early night (up to 3:40) in the city. That's my advice

If you're flying with one of the following airlines - Iberia, Spanair, Air Europa, Alitalia, Pluna or Portugalia - you can check your luggage in in the Muevos Ministerios metro station up to 24 hours in advance. That's real check-in: ie they take your bags off you there and then without you having to go to the airport and they put them on your flight for you. As long as you aren't including live animals!

Here's what the airport authority says on their webpage:

"Check-in
There are 34 ticket counters at the Nuevos Ministerios transportation hub, where travellers can check in for flights with Air Europa (except Buenos Aires), Iberia (except air shuttle flights), Spanair, Alitalia and Pluna.

At Nuevos Ministerios, Iberia runs check-in services from 6:30 am to 10:30 pm; Air Europa from 6:30 am to 9:30 pm; Spanair from 7:30 am to 12:30 am and from 3:30 pm to 8:30 pm, from Monday to Friday; from 7:30 am to 12:30 am on Saturdays; and from 3:30 pm to 8:30 pm, on Sundays. Alitalia runs check-in services from Monday to Friday and on Sundays from 8:00 am to 6:30 pm and on Saturdays from 9:00 pm to 5:00 pm. Lastly, Pluna runs check-in services on Mondays, Tuesdays and Fridays from 12:00 pm to 10:00 pm. Aena recommends you confirm these timetables with your airline.

Check-in is a minimum of two hours prior to the scheduled flight for passengers with baggage and one hour prior for passengers without baggage. With Iberia, the maximum time for check-in prior to the flight is 24 hours. With Spanair, Air Europa y Portugalia, passengers with baggage may only check in on the same day of the flight from 6:30 am. Passengers without baggage may check in up to twenty hours before departure.

At Nuevos Ministerios, checking in baggage larger than 900 x 800 x 450 mm or heavier than 50 kilos is not permitted. It is also prohibited to check baggage that must pass customs inspection, special baggage (bicycles, surfboards, and so on) nor live animals."

I just came from the airport, asked the guy there and luggage storage is open 24 hours. No problem on collecting your bags during the night.

Just wanted to thank Nativo, Barry and Castiza for their replies- very helpful! You guys are great. Nativo: I like your suggestion but my last night in Madrid will be a Sunday so I'm not sure if there will be much to do in Madrid at 2-3 Sunday morning. If I'm wrong, let me know!