Smith-Magenis syndrome occurs in anywhere from 1 in 15,000 to 1 in 25,000 births. It is thought that many instances of Smith-Magenis remain undiagnosed. The disability is caused by a deletion of part of the short arm of chromosome 17.
Smith-Magenis causes mild to moderate intellectual disability, delayed speech and language development, sleep disturbances and behavioral problems. Physical characteristics of those with this disability include a broad, square-shaped face with deep-set eyes, full cheeks and a somewhat prominent lower jaw. They also tend to short stature and scoliosis of the spine.
Those with Smith-Magenis tend to experience a reversal of the biological clock that keeps them awake at night and sleepy during the day. Research and treatments into sleep disturbances has been successful and have allowed those with this disability to return to a normal sleep cycle. Other characteristics can be seen here.
For additional information and resources, see Information from the NIH website or PRISM, Parents and Researchers Interested in Smith-Magenis.