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: Two groups of supine subjects were studied under placebo - controlled conditions , one during the night , when sleeping ( n = 7 ) and the other at daytime , when awake ( n = 6 ) .
### Output Text:
Two groups of supine subjects were studied under placebo - controlled conditions , one during the night , when sleeping ( n = 7 ) and the other at daytime , when awake ( n = 6 ) . 