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The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
If all the neurons in the human body were lined up, they would stretch more than 600 miles.
An identified risk factor for osteoporosis is the intake of excessive amounts of vitamin A. Dietary intake of approximately double the recommended daily amount of vitamin A, by women, has been shown to reduce bone mineral density and increase the chances for hip fractures compared with women who consumed the recommended daily amount (or less) of vitamin A.
In most cases, kidneys can recover from almost complete loss of function, such as in acute kidney (renal) failure.
The first-known contraceptive was crocodile dung, used in Egypt in 2000 BC. Condoms were also reportedly used, made of animal bladders or intestines.