I’ve never really heard one good reason for not wearing the Brown Scapular whilst asleep or in the shower. Are you any less in Our Lady’s service as you sleep? Are you somehow more resilient against the wiles of the devil when you’re showering?
I sleep with my scapular on and I have spares so that showering is not a problem. There has never been any discomfort. I haven’t been strangled in my sleep by the scapular.
The Blessed Virgin does not sleep in her service to us. I for one wont sleep in my service to her.
And as for the children of Fatima, they were not told to sleep without their scapular. They were told that they could relax on the severe penances they gave themselves involving a cord rope.