826. _The close student and the growing child need more sleep than the idler or the adult._ As steep is the natural repose of all organs, it follows that the more the brain and other organs of the system are employed, the more repose they require. The organs of the child, beside sustaining their proper functions, are busy in promoting its growth. This nutritive process is attended with a certain degree of exhaustion. The impaired health of children often results from a disregard of this principle. But, on the other hand, an excess of sleep produces feebleness, by preventing the proper exercise of the mind as well as the body. 827. _The length of time the brain may be advantageously used, is modified by many circumstances._ The power of the brain in different persons to endure action, is various. This is modified by its primary character; by development and age; by habits of action; by the health of the cerebral organ and general system; by the moral feelings and other conditions. -=-=-=-=-=-=-=-=-=-=-=-= 825. Why should we engage in intense study in the early part of the day? 826. What persons require the most sleep? Why? 827. What is said relative to the length of time that the brain can be advantageously used? Give a condition that modifies the amount of mental labor. -=-=-=-=-=-=-=-=-=-=-=-= 828. The primary physical organization of some individuals is such, that they are enabled to endure with impunity an amount of mental labor that would disorder, if not destroy functionally, the cerebral organ of others differently constituted. Napoleon Bonaparte was of this number. There can be no fixed period for mental labor, that may be adopted as a rule for all persons whose systems are maturely developed. Much less is there a proper definite period for study, that is applicable to all children. _Observation._ The practice of retaining pupils of all ages, from five to twenty years, in the school-room the same period of time, for the purpose of study, is not predicated upon any law of physiology. An exercise of three hours, with one or two recesses of ten minutes each, may profit the eldest class; two hours with a recess of ten minutes, the middle class; while one hour, or one hour and a half, with one recess, would be as long a period as the youngest pupils should be retained in the study-room at one session. 829. A person who is accustomed to muscular exertion will endure a longer period of physical toil than one who is not inured to it. So it is with mental labor. If the brain has been habituated to mental action and profound study, it will not be so soon fatigued as when not accustomed to such exertions; consequently, an amount of mental labor may be performed with impunity at one time, that would exhaust and cause serious disease of the cerebral organ at another.