Table 3.

Levels of Evidence Adapted from Oxford Centre for Evidence-Based Medicine (May 2001)

Level	Risk/ Assessment	Treatment
1	Validating1 cohort with well-validated reference standards2	High quality randomized controlled trial (RCT) on well-characterized subjects or patients
2	Smaller or “exploratory” cohort study or one that has incompletely validated reference standards2	Cohort study or flawed clinical trial (e.g., small N, blinding not specified, possible non-random assignment to treatment, incompletely validated reference standards2)
3	Case control study or cross-sectional survey	Case control study
4	Case series (and poor quality cohort and case control studies)	Case series (and poor quality cohort and case control studies)

^1.Validating studies test the quality of a specific diagnostic test, based on prior evidence.

^2.Reference standards: PSG, sleep logs, actigraphy, phase markers, validated self-reports.

The Oxford system defines four levels of evidence, and appends each level with an “a” if the evidence is based on a systematic review, or a “b” if it refers to a single study. Because we did not find any systematic reviews (only individual reports), we dropped the “b” and indicated only the numerical level of evidence. Papers that were considered important background citations are included in the bibliography without an evidence grade.