Genuine art music is never soporific ('soothing'). That is simply because the over-riding feature of this music is 'newness'. However, if one becomes very familiar with a work, this familiarity eventually robs the music of this feature. That is why, for art music to thrive, it is vital that audiences are constantly supplied with 'new' music.
Orchestral administrators, eager for financial gains, often say that, to fill an auditorium with listeners, it is best to stick to the 'familiar' repertory. No doubt, in the short term, they are correct. In the long term, of course, they are responsible for the potential death of art music itself. [Would you genuinely enjoy an old warhorse such as 'Handel's Messiah'? Would you go because you were eager to hear it, or would it be because you didn't want to offend some of the performers?]
This 'familiar repertory' of course, is usually wonderful music. The fault is not the music, but the fact that it has been performed so often, that many people eventually become bored with it because the essential art music ingredient - newness - has been lost.
In my experience, however, it is extremely rare for anyone in an audience to actually fall asleep. If it happens, it is likely to be a pretender, someone wishing to be known as a lover of fine music. In order to maintain the involvement of genuine music lovers, astute performers strive to find something 'new' in their performance. [ In my view, it would be better to supply 'new' music, but that is only my opinion.]
What you may well have noticed, and misinterpreted as slumber, is the common habit of listening to profound music with eyes closed. I do it, and so do many like me.
If I wanted to use music as a sleep inducer, I would choose music that I had once loved, but had heard too often. I would listen to it alone, not in a concert auditorium. Or, if I really needed sleep, I would choose 'Bella's Lullaby' or 'music' of that ilk.