The suppression of melatonin by light at night (LAN) has been associated with a disruption of SCN function and biological processes. This study aimed to explore the impact of melatonin on glucose and lipid metabolism before and after a late evening meal. Nine healthy male participants (26 ± 1.3 years, BMI 24.8 ± 0.8 kg/m2 (mean ± SD) were randomly categorised into a three-way cross over design protocol: light (> 500 lux) (LS), dark (<5 lux) + exogenous melatonin (DSC), and light (> 500 lux) + exogenous melatonin (LSC). All participants were awake in a semi-recumbent position during each clinical session, which started at 18 00 hours and ended at 06:00 hours the following day. The meal times were individualised according to melatonin onset estimated from participants 48h sequential urine collection. The administration of exogenous melatonin was conducted 90 minutes before the evening meal. Saliva and plasma samples were collected at specific time points to analyse the glucose, insulin, NEFAs, TAGs, cortisol, and melatonin levels.

Participants demonstrated a significant reduction in postprandial plasma glucose, insulin, and TAGs levels in the presence of melatonin (LSC and DSC) compared to LS (p= 0.002, p = 0.02, and p = 0.007, respectively). Pre-prandial plasma NEFAs were significantly lower in LS than DSC and LSC as melatonin rose (p < 0.001). Exogenous melatonin administrated before an evening test meal improved glucose tolerance, insulin sensitivity, and reduced postprandial TAGs. This study could have implications for shift workers who may have lower melatonin levels at night due to light suppression.