In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Melatonin affects the circadian sleep / wake cycle , but it is not clear whether it may influence drug - induced narcosis .
### Output Text:
<span class="chemical">Melatonin</span> affects the circadian sleep / wake cycle , but it is not clear whether it may influence drug - induced narcosis . 