Given the following context:

Nielsen's two operas are very different in style. The four-act Saul og David (Saul and David), written in 1902 to a libretto by Einar Christiansen, tells the Biblical story of Saul's jealousy of the young David while Maskarade (Masquerade) is a comic opera in three acts written in 1906 to a Danish libretto by Vilhelm Andersen, based on the comedy by Ludvig Holberg. Saul and David received a negative press when it was premiered in November 1902 and did no better when it was revived in 1904. By contrast, in November 1906 Masquerade was a resounding success with an exceptional run of 25 performances over its first four months. Generally considered to be Denmark's national opera, in its home country it has enjoyed lasting success and popularity, attributable to its many strophic songs, its dances and its underlying "old Copenhagen" atmosphere.Nielsen wrote a considerable number of choral works but most of them were composed for special occasions and were seldom reprised. Three fully-fledged cantatas for soloists, orchestra and choir have, however, entered the repertoire. Nielsen composed Hymnus amoris (Hymn of Love), Op. 12 (1897) after studying early polyphonic choral style.  Writing in the newspaper Dannebrog, Nanna Liebmann referred to the work as "a decisive victory" for Nielsen, and Angul Hammerich of Nationaltidende welcomed its improved clarity and purity. But the Berlingske Tidende reviewer H.W. Schytte thought Nielsen had been pretentious presenting the lyrics in Latin rather than Danish. Søvnen (The Sleep), Op. 18, Nielsen's second major choral work, sets to music the various phases of sleep including the terror of a nightmare in its central movement which, with its unusual discords, came as a shock to the reviewers at its premiere in March 1905. Fynsk Foraar (Springtime on Funen), Op. 42, completed in 1922, has been cited as the most Danish of all Nielsen's compositions as it extols the beauty of Funen's countryside.

answer the following question:

What composition premiered in March 1905? 

Søvnen