The human spine is not, as one may think, a rigid structure. If you stand up during much of the day, your spine will compress slightly to the point where you might lose a few inches off your total height. During sleep, your body relaxes and your spine 'extends' again, as no muscles or weight are pressing down on it, which would make you slightly taller when you wake up than when you go to sleep.
OK it really is hard not to tease you, but on a serious note, it would make sense as the solfeggio frequencies are working on you physically, and may cause a deeper relaxation than normal sleep. Make sure though that you are listening to the original frequencies, not the recent ones that were changed by Vatican II. Maintaining the height may be needed to strengthening those muscles that support the spine and maybe listening to the frequencies more than just at night. I play them for my plants and animals while I am gone too.