Table 3. Risk of bias within included studies.

Risk of bias tool	Risk of bias criteria	K. 1978	L. 1989	V.P. 1995	Z. 2000	V. 2012	S. 2013	S. 2014	Z. 2014	K. 2015	F. 2016
JBI checklist for analytical studies	Clear inclusion criteria	+	+	+	+	+	+	+		+	+
	Study description	+	+	+	+	+	+	+		+	+
	Consistent light conditions	–	–	+	+	+	–	–		–	–
	Consistent melatonin measurement protocol	+	–	+	+	+	+	–		+	+
	Clear description of confounders	–	–	–	+	–	–	–		–	+
	Adjustment for confounding variables	–	–	–	+	–	–	–		–	+
	Sufficient melatonin measurements	–	–	–	+	–	–	–		+	–
	Outcome analysis	–	–	–	+	–	–	–		–	–
	AHRQ Score	L	L	L	H	L	L	L		L	L
Cochrane Risk of Bias Tool	Random sequence generation						–	?	?	+
	Allocation concealment						–	?	?	+
	Selective reporting						+	+	+	+
	Other bias						+	+	+	+
	Blinding of participants/assessors						–	+	+	+
	Blinding of outcome assessment						–	+	+	+
	Incomplete outcome data						–	?	+	+
	AHRQ Score						L	L	F	H

Studies are denoted using the main initial of the primary author and the date of publication. The risk of bias tools is given on the far left with the accompanying criteria. Boxes are shaded according to the following scale: meets full criterion (+), does not meet (–), not applicable (grayed out), or unsure (?). Conference proceedings and abstracts (Kostovski et al.⁵³, Vaughan et al.⁵⁴) were not analyzed for risk of bias given the lack of detail surrounding their study design. The study design by Zeitzer et al.⁴⁸ was excluded (grayed out) from the JBI checklist given limited information regarding melatonin measurement and analysis, as the absence of nocturnal melatonin was used only as an inclusion criterion for study enrollment. For each checklist, studies were given an AHRQ score of L (low quality), F (fair quality) or H (high quality).