Melatonin
<p><b >Melatonin</b> <a href="http://upload.wikimedia.org/wikipedia/commons/5/5f/Melatonin-pronunciation.ogg" title="Listen" ><img src="/cde75539-e007-54bf-859d-0584270f4d74.png"/></a><sup ><a href="http://en.wikipedia.org/wiki/File:Melatonin-pronunciation.ogg" title="File:Melatonin-pronunciation.ogg" >i</a></sup><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English" title="Help:IPA for English" >/</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English" >ˌmɛləˈtoʊnɪn</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English" title="Help:IPA for English" >/</a>, also known chemically as <b ><i>N</i>-<a href="http://en.wikipedia.org/wiki/Acetyl" title="Acetyl" >acetyl</a>-5-<a href="http://en.wikipedia.org/wiki/Methoxy" title="Methoxy" >methoxy</a><a href="http://en.wikipedia.org/wiki/Tryptamine" title="Tryptamine" >tryptamine</a></b>,<sup id="cite_ref-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Melatonin#cite_note-1" >[1]</a></sup> is a <a href="http://en.wikipedia.org/wiki/Hormone" title="Hormone" >hormone</a> found in animals, plants, and microbes.<sup id="cite_ref-Caniato2003_2-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Melatonin#cite_note-Caniato2003-2" >[2]</a></sup><sup id="cite_ref-Paredes2009_3-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Melatonin#cite_note-Paredes2009-3" >[3]</a></sup> In animals, circulating levels of melatonin vary in a daily cycle, thereby allowing the <a href="http://en.wikipedia.org/wiki/Entrainment_(chronobiology)" title="Entrainment (chronobiology)" >entrainment</a> of the <a href="http://en.wikipedia.org/wiki/Circadian_rhythm" title="Circadian rhythm" >circadian rhythms</a> of several biological functions</p>