Sleeping Animals

Other sleep disorders
Sleep disorders include narcolepsy, periodic limb movement disorder (PLMD), restless leg syndrome (RLS), and the circadian rhythm sleep disorders. Fatal familial insomnia, or FFI, is an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms; ultimately sufferers of the disease stop sleeping entirely, before dying of the disease.
Older people are more easily awakened by disturbances in the environment and may to some degree lose the ability to consolidate sleep.