Rather than taking bookings for train seats 24/7, as per usual on websites, they've decided that if you want to book a train between 11.30pm and 7am Spanish time, you're out of luck because they're closed for the night.
If you try to book a train ticket between 11.30pm and 7am you're given this response: "E0019 - Aplicación VBI no disponible. Nuestro horario es de 07:00 a 23:30 horas (Hora peninsular española)" which, when run through altavista's Babel Fish translation, says "Application VBI not available. Our schedule is from 07:00 to 23:30 hours".
The trains, however, seem to run at all hours, so why exactly you can't book between 11.30 and 7 seems rather mysterious. We can only assume that website administration in Spain, where shift work is outlawed, is rather primitive, and they have a team of well-trained but drowsy Spaniards working on the site from 7 to 11. µ