Most citations in PubMed are for journal articles. However, you may limit your retrieval based on the type of material the article represents. Use the Filters on the Results page sidebar and look at the Article Types checklist which contains a list of frequently searched publication types.
For example, choose Randomized Controlled Trial or Clinical Trial or Meta-Analysis from the list.
Enter your search terms and evidence-filtered citations will appear under Clinical Study Categories. Systematic Reviews or Medical Genetics. The Clinical Queries link is found on the PubMed home page or under the More Resources drop-down at the top of the Advanced Search page.
The resulting retrieval in PubMed Clinical Queries can be further refined using PubMed's Filters, e.g., English language, humans.
Many abstracts that are added to PubMed include section labels such as BACKGROUND, OBJECTIVE, METHODS, RESULTS, and CONCLUSIONS. These 'structured' abstracts appear in many different article types such as review articles, original research, and practice guidelines and facilitate skimming of citations for relevance and specific information such as research design within the Methods section. The presence of structured abstracts in citations are a searchable feature in PubMed. To limit to citations containing structured abstracts, include the term hasstructuredabstract in the search box.
For example: valerian AND sleep AND hasstructuredabstract