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: On day 1 , all participants exhibited a typical cortisol profile , though light users had significantly elevated levels pre - bed .
### Output Text:
On day 1 , all participants exhibited a typical <span class="chemical">cortisol</span> profile , though light users had significantly elevated levels pre - bed . 