The circadian rhythm is influenced by light-sensitive retinal proteins that trigger signals to the

a. suprachiasmatic nucleus.
b. thyroid gland.
c. sleep spindles.
d. dopamine reward system.

a. suprachiasmatic nucleus.

Explanation:

The circadian rhythm is an internal process that regulates daily cycle (sleep/wake conditions through the 24 hours). This process is performed by circadian clock which is located in hypothalamus, in the suprachiasmatic nucleus. It has been shown that when the suprachiasmatic nucleus is destroyed, regular sleep–wake rhythm is lost.

The suprachiasmatic nucleus sends information to the pineal gland, which secretes hormone melatonin (regulator of  sleep–wake cycle).

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