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: Participants underwent polysomnographic sleep recordings on days 1 to 3 , 7 to 9 , and 14 to 16 ( first , second , and third weeks of abstinence ) .
### Output Text:
Participants underwent polysomnographic sleep recordings on days 1 to 3 , 7 to 9 , and 14 to 16 ( first , second , and third weeks of abstinence ) . 