What is the evidence linking endogenous melatonin to sleep cycles?
A study was considered relevant to this question of the review if it involved an intervention that altered endogenous melatonin levels or the sleep cycle, such as a manipulation of light/dark exposure or the sleep schedule, and it examined either melatonin levels in blood, urine, saliva or CSF or an aspect of the sleep cycle, depending on which intervention was used. That is, if the study intervention was designed to manipulate endogenous melatonin, then it was necessary that the study examine the effect of this manipulation on an aspect of the sleep cycle, and vice-versa. These criteria allow for an understanding of the relationship between endogenous melatonin and the sleep cycle via an assessment of the effect of manipulation of one variable on the other. Forty-four studies were relevant to this question of the review. The overall quality scores, according to the Down’s and Black Checklist, ranged from eight to 23 on a 29-point scale; most studies had a score between 16 and 20, one
