What percent of people in the developed world suffer from sleep disorders?

Huge. Sleep apnea is common and affects millions. Insomnia is also all too commond. Most of these disorders are in fact diet related because high calorie malnutrition is widespread.
Sleep. Usually incidence of insomnia is 10 to 15 % acute or chronic, hypersomnia, sleep apnea is 5 % or so and then the others sleep wake delayed syndrome etc varies.