In human plasma 61-85% of melatonin is weakly bound to proteins, and there is a close relationship between circulating free and salivary melatonin levels. (6) Melatonin enters saliva from blood either by passive diffusion or active transport. (7) Melatonin is measurable in saliva, and the acrophases of saliva and plasma melatonin rhythms are significantly correlated. (8,9) Plasma and salivary melatonin concentrations increase when moving from a supine to a standing position, and decrease when these positions are reversed, due to changes in plasma volume. (10) Inflammatory processes such as periodontitis trigger an increase in plasma melatonin, which then increases melatonin levels in the oral cavity, where it may increase antioxidant protection. (11)​

Melatonin and the Endogenous Clock

Due to a strong correlation with serum melatonin concentrations, analysis of salivary melatonin effectively enables researchers to design non-invasive studies that assess circadian timing and evaluate the degree of circadian phase abnormalities in association with major diseases, conditions, administration of pharmaceutical compounds, seasonal affective disorder, jet-lag and night shift work, etc. It also pertains to the clinical aspect of improving diagnosis and treatment of irregular sleep-wake disorders via chronotherapy using exogenous melatonin or altered light exposure. Research of melatonin dysregulation affects multiple body-wide systems and often results in chronic sleep deficits with symptoms that can be misdiagnosed as depression, attention deficit hyperactivity disorder, fibromyalgia, or chronic fatigue.

DLMO & Melatonin Onset Analysis

With saliva as a biological sample, researchers can non-invasively monitor melatonin levels without an overnight sleep clinic stay (polysomnogram). This eliminates the rhythm perturbation caused by a clinical sleep center laboratory, can be performed in 4-6 hrs (as opposed to 24 hrs), and offers the convenience of collecting samples at home. Salivary DLMO is determined by measuring salivary melatonin levels over several time-points prior to and just after habitual sleeptime. Calculating the time that the nocturnal melatonin concentration is significantly elevated from daytime samples (+/- 2 deviations from baseline levels)- DMLO is the most sensitive and direct index for identifying an individual’s biorhythm which may be entrained (synchronized) to a 24 hr light/dark cycle or can be in a free-running state. A misaligned DMLO may indicate delayed sleep phase syndrome (DSPS) or advanced sleep phase disorder (ASPD) and its relationship to various metabolic conditions, diseases, disorders and genetic and environmental components are of significant interest in current research.