**4. Melatonin content in foods**

The melatonin hormone exists in different types of food, although its content can be very different from one to another, being in nuts and some medicinal plants where it has been found the most [62].

In edible plants, the fruits seem to be the part that contains the least melatonin, while the roots and leaves contain the most [63]. It also depends on the environment where the plants grow, how they are cultivated, temperature, sun exposure, agricultural treatments, etc. [64].

In animals, melatonin has been found mainly in eggs and fish and to a lesser extent in meat. Melatonin has been found in human breast milk, and that of other animals, to vary with circadian rhythms, being lower during the day and higher at night [65, 66]. Regarding plants, melatonin has been found in cereals, although they are still being investigated in different species such as corn, black rice, barley, and oats, among others [67]. In fruits, melatonin has been found in grapes [68], cherries [69], and strawberries [68], other fruits seem to have relatively lower levels of melatonin.

We found melatonin in different vegetables, being undetectable in beets and potatoes [70], and instead we found higher concentrations in mushrooms, tomatoes, and peppers [68]. It has been found in seeds and legumes with relatively high levels; in addition, it has been seen that in the germination process it can increase melatonin levels. Highest levels of melatonin have been found in walnuts [62].

Melatonin has also been searched for in different medicinal plants and high values have been found, above 1000 ng/g [13], for example, it is the case of St. John's Wort, (*Hypericum perforatum*) [71].
