Night buses: 'buhos'

"If you need a lift after 23.30 pm you'll have to rely on either the taxis or the 'buhos' -Madrid night buses. There are over 20 'buhos' bus lines. All start at Plaza de Cibeles (on map C2-[d4]), at 24.00 pm leaving on weekdays every 35 minutes until 3.20 am and then every 70 minutes all night long. At weekends and Fridays they run all night long every 20 minutes.

There is also a 'Metrobuho' service at weekends and Fridays, bus lines that follow the metro lines paths and have the same frecuency as the 'buhos'."

http://www.softdoc.es/madrid_guide/transport/buses.html