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: The effect of modafinil interacted with the abstinence week and was associated with longer total sleep time and shorter REM sleep latency in the third week of abstinence .
### Output Text:
The effect of <span class="chemical">modafinil</span> interacted with the abstinence week and was associated with longer total sleep time and shorter REM sleep latency in the third week of abstinence . 