Melatonin is involved in biological rhythms and photoperiodicity

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The pineal gland is situated between the two hemispheres of the brain and is connected to the brain by a little stalk. It produces the hormone melatonin from the amino acid tryp-tophan. The release of melatonin by the pineal occurs in the dark and therefore marks the length of the night. Exposure to light inhibits the release of melatonin.

In various vertebrates, melatonin is involved in biological rhythms, including photoperiodicity—the phenomenon whereby seasonal changes in day length cause physiological changes in animals. Many species, for example, come into reproductive condition when the days begin to get longer (Figure 42.13). Humans are not strongly photoperiodic, but melatonin in humans may play a role in entraining daily

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Winter (long nights)

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Melatonin Photoperiod Seasonal

42.13 The Release of Melatonin Regulates Seasonal Changes

(a) Melatonin is released in the dark and inhibited by light exposure. The duration of daily melatonin release thus changes as day length (photoperiod) changes, inducing dramatic seasonal physiological changes in some animals. (b) In winter,these Siberian hamsters are white and nonreproductive. In summer, they are mottled brown and breed.

42.13 The Release of Melatonin Regulates Seasonal Changes

(a) Melatonin is released in the dark and inhibited by light exposure. The duration of daily melatonin release thus changes as day length (photoperiod) changes, inducing dramatic seasonal physiological changes in some animals. (b) In winter,these Siberian hamsters are white and nonreproductive. In summer, they are mottled brown and breed.

biological rhythms to the daily cycle of light and dark. We will learn more about biological rhythms in Chapter 52.

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