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: For comparison of sleep architecture variables , 12 healthy comparison participants underwent a single night of experimental polysomnography that followed 1 night of accommodation polysomnography .
### Output Text:
For comparison of sleep architecture variables , 12 healthy comparison participants underwent a single night of experimental polysomnography that followed 1 night of accommodation polysomnography . 