Adult sleep music is not a lullaby or a meditation track
A lullaby rocks a child to sleep with a singable tune and a name in the chorus; this generator drops the tune and the rocking meter entirely and asks for a beatless, wordless bed instead. Meditation music is built to be noticed gently, an anchor for attention during a session; sleep music is built to be forgotten, fading into the dark while the mind lets go, so it should sit lower and duller than either.
Ask for beatless, or if you want a faint pulse, keep it under 60 BPM, slower than almost every other genre. Subdued pads, a low sustained drone and a quiet nature layer like rain or distant ocean do the work; leave out bright bells, plucked strings or any phrase that repeats like a melody, since a hook is the one thing that keeps a tired brain listening instead of drifting.