This sentence is grammatically correct but it is not adverb phrase it is Syntax
The serotonin is a chemical neurotransmitter in the brain responses to keep the "happy mood" in humans, so-called, the "the molecule of happiness". Low levels of serotonin may produce serious problems like depression, anxiety, memory low-self esteem, irritation and so on. When a lover has a low serotonin level in his brain, his mood and general behavior can be affected in great ways. Young people may not be aware of the great-negative consequences of the decreased levels of serotonin and a treatment should be carry out. Depression and anxiety can lead to terrible outcomes in someone's life.
Ccording to the MLA Handbook, when you cite a source in the text of your paper, the citation should interrupt the text as little as possible; you want to lead the reader to the correct long-form citation in your Works Cited page with a minimum of fuss (116). There are two ways to do this - with a signal phrase, meaning the in-text citation will have only the page number(s), or with a parenthetical citation, meaning the in-text citation will include the author's last name (or the title, for an authorless work) and the page number(s) (Howard 289).
Signal Phrase
Format: Signal phrase, "quote" or paraphrase (page number).
The first sentence of the first paragraph on this page contains a signal phrase - "According to the MLA Handbook..." - with a page number in parentheses at the end of the sentence. The signal phrase lets your reader know that you are paraphrasing or quoting an idea from someone else's work. If your paper deals with a particular work of literature, or if you are relying heavily on the work of one or more sources, a signal phrase introducing the source is recommended.
Note that if you are quoting a source, in-text citation always comes after the closing quotation mark.
Examples of a Signal Phrase
In her work Pride and Prejudice, Austen makes the famous observation that "it is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife" (3).
Duhigg argues that we can change our habits, but because they are deeply ingrained into the brain, it can be a struggle to do so (20).
