Definition
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When the mind and sense organs get exhausted, they withdraw themselves from their objects and the individual sleeps.
Sleep is nothing but location of mind in a place unconnected with sensor and motor organs.

Proper sleep
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The sleep caused by nature of night is excellent sleep. It keeps every one lively, nourishes them like mother.
When tamas increases the person sleeps. When satva qualities increases person awakens. Naturally the tamas & satva qualities alternately are held responsible for sleeps and awaiening.

Proper way to sleep
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Over a comfortable bed in home and proper time at which sleep is accustomed, one should go to sleep.

Relation of sleep with health
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Happiness, misery, nourishment, emaciation, strength, weakness, sterility, virility, knowledge, ignorance, life and death, all are dependent on proper or improper sleep.
Untimely sleep and excessive sleep take away both happiness & longevity like kala ratri ie fierceful night. At the same time is properly enjoyed, they bring about happiness, longevity, to human being as real knowledge ie satya buddhi brings about siddhi to yogi (monk).

Relation of sleep with nourishment and malnourishment of the body.
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Awakening at the night increases dryness in the body. Day sleep increase unctuousness in the body and sleeping in sitting posture during day time does not increase unctuousness or dryness.
Like the proper diet, proper sleep is essential for maintenance of the body, proper sleep increase weight, induces obesity where as less sleep produce emaciation.
Sleep which is untimely, excessive or very less destroys the happiness and longevity of life similar to the all destructive night at the end of the wold.
Sleep is vital for all organisms. Its improper practice may be even fatal.