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Q.
A hormone responsible for normal sleep-wake cycle is
Chemical Coordination and Integration
Solution:
Melatonin is a hormone secreted by pineal gland. Immediately above the optic chiasma (in the brain) in a nucleus, are present melatonin receptors that react to this hormone and synchronise the nucleus to the $24$ hrs day/night rhythm, thus informing the brain when it is day and when it is night.