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: Significant between group differences were also observed in afternoon cortisol levels and in overall cortisol secretion across the day .
### Output Text:
Significant between group differences were also observed in afternoon <span class="chemical">cortisol</span> levels and in overall <span class="chemical">cortisol</span> secretion across the day . 