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: As the relative increase in PRL secretion expressed as a percentage of the mean did not significantly differ between the night and day time studies ( 46 + / - 19 % vs 34 + / - 10 % ) , it can be concluded that the observed sleep disturbance did not interfere with the drug action per se .
### Output Text:
As the relative increase in PRL secretion expressed as a percentage of the mean did not significantly differ between the night and day time studies ( 46 + / - 19 % vs 34 + / - 10 % ) , it can be concluded that the observed sleep disturbance did not interfere with the drug action per se . 