Theory:

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Human hormones and their secretions
 
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Secretions of different glands
 
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The 24hr circadian cycle in human - pineal gland
Reference:
https://lh3.googleusercontent.com/m1-Q-baBF6Oo_9zpQMLQFXrJoZXyOpytcsfrbm4y5mqQuVbDSvhJ7JFjsPrDTs850uX0xNKc5cqXXQ--nAezG9CtKOPRgHGx7b2Cawr87OX2b5H03obsH21f9wrW_BglYXgyM6DufPI8CXp-W-4v4y6EErLNpdsYV8s8XQ-6WG1Y7DFPb410mt0u2DPcnwUrVP_JmHtl_zWaMIDAhG43JEpuBNUYzHaM=w1568-h796
https://commons.wikimedia.org/wiki/File:Endocrine_central_nervous_ca.svg#/media/File:Endocrine_central_nervous_en.svg
https://commons.wikimedia.org/wiki/File:The_master_circadian_clock_in_the_human_brain.jpg