It is rather unusual for a journal to not give information about article length, document type, etc., in the instructions list for authors. I think the best thing you can do is to check the published articles in the journal and get a rough idea of the article length and formatting style. Although many journals recommend following the APA style, it is best to follow the pattern of their published articles, at least, for basic formatting requirements and the style of references and citations. Here are a few general tips on the APA style that might help you:
- Articles should be typed, double-spaced with 1" margins on all sides.
- Articles should be in a readable font, preferably Times New Roman 12 point.
- A page header or "running head" should be added at the top of every page. The running head should be a shortened version of the article title and should not exceed 50 characters.
- The title page should contain the title of the paper, the author's name, and affiliation.
- The abstract should be around 150-250 words long.
- In-text citations should be provided as well as detailed references in a separate references section.
The APA style guide provides a tutorial that will help you to learn how to use this style. http://www.apastyle.org/learn/tutorials/basics-tutorial.aspx
After going through the APA tutorial and some of the published articles in Behavioral Sleep Medicine, if you still feel that you are unclear about the details, you can write to the journal editor requesting more information.