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: Morning dipping , assessed by the fall in peak flow overnight , was significantly reduced in the periods when either active drug was taken , whereas no difference was noticed during the placebo administration .
### Output Text:
Morning dipping , assessed by the fall in peak flow overnight , was significantly reduced in the periods when either active drug was taken , whereas no difference was noticed during the placebo administration . 