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I'm wondering what produces the feeling of hunger in humans. Checking Wikipedia revealed that leptin and ghrelin are two hormones involved. I've also read that the digestive system produces its own melatonin.

Because melatonin is related to circadian rhythm and the biological clock,Is melatonin involved in feeling hungry? Is there a circadian pattern to the levels of leptin and ghrelin in humans?

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Many hormones aside of melatonin show circadian rhythms. I doubt melatonin is directly involved in this, but I would bet cortisol is for instance. –  nico Nov 1 '12 at 9:28
    
This question demands an answer that is tissue specific. Leptin is produced by skeletal muscle as well as adipose. Production of a hormone or signaling molecule may be under circadian control in one tissue but not another. In addition, a complete answer will talk about aging as circadian rhythms in peripheral tissues break down with aging. –  Larry_Parnell Nov 1 '12 at 16:26
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Yes, absolutely. A major focus of understanding obesity, diabetes and other metabolic disorders are targeted to understanding the circadian (and other cyclical nature) systems in neurobiology and endocrinology. I can speak within the study of diabetes, there is an observed diabetic "dawn effect" in which there is an early morning (dawn) spike in blood glucose. The exact mechanism for this spike in blood glucose is not fully known, but the general circadian nature is intriguing and would suggest a dominant hormonal cause.

Some other review articles may be found here. An overall review of homeostatic balance as it relates to the central nervous system and apetite regulation. The second review more closely focuses on the biochemistry and endocrinology of central and peripheral regulation on food intake and physical activity.

  1. Berthoud HR, Morrison C. The brain, appetite, and obesity. Annu Rev Psychol. 2008;59:55-92.

  2. Lenard NR, Berthoud HR. Central and peripheral regulation of food intake and physical activity: pathways and genes. Obesity (Silver Spring). 2008 Dec;16 Suppl 3:S11-22.

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I found additional information on the subject. Apparently Serotonin is also involved in feeling hunger and the motivation to eat. It also follows a diurnal/circadian pattern as described below:

Serotonin as as Gauge of food availability (appetite)

The expression of 5-HT2C receptors in the hippocampus follows a diurnal rhythm,[13] just as the serotonin release in the ventromedial nucleus, which is characterised by a peak at morning when the motivation to eat is strongest.[14]

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