Biological organisms absorb and store radioactive isotopes incorporated in the tissues that make up the organisms.

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"We aren't sure why activity primes your body for sleep so well, but it's likely a combination of factors, including lowering your core body temperature, increasing the sleep-promoting hormone melatonin and supporting a biological need to restore energy levels and repair cells and tissues when you sleep, " Cardinal says.

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Topping up the supply of healing fluid might be done by mimicking another biological system the network of blood capillaries that supplies living tissues with the stuff they need to thrive.

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