(a) sleep disorders/many ailments.
(b) It is important for physicians to 'wake up' to sleep disorders because sleep disorders are behind many ailments. If sleep history is considered in evaluating a patient, it will help greatly in diagnosing the ailments.
(c) By 'sleep deficit I mean the sleep that should have been there but was not had due to various reasons. The amount of sleep required to rejuvenate people varies from person to person. It is so because the physiological aspects and living conditions are different from one to another.
(d) The writer refers to the early to bed principle because it is just the best for a healthy person. However, it may not be possible today.