Optimising supplemental lighting protocols for tropical controlled environment agriculture remains challenging due to limited photoperiod-specific research. This study investigated LED timing effects on mustard greens (Brassica juncea L.) growth under Vietnamese greenhouse conditions. Four treatments were evaluated: LED_0 (control), LED_18-22 pm (evening supplementation), LED_22 pm - 2 am (night supplementation), and LED_2-6 am (early morning supplementation). Custom LED modules with 50W Chip-on-Board technology provided 60-70 μmol.m⁻².s⁻¹ PPFD at 0.5m distance. Plants were grown in commercial Tribat soil with automated irrigation maintaining 70-80% moisture. Morphological parameters, SPAD chlorophyll index, and biomass allocation were assessed over 42 days. Evening LED supplementation (18-22 pm) demonstrated superior performance, achieving a 70.7% increase in plant height (61.64 vs 36.10 cm), a 58.0% enhancement in total fresh weight (20.65 vs 13.07 g), and a 26.2% improvement in leaf number compared to controls. However, this treatment exhibited morphological trade-offs with a 11.8% reduction in leaf width and 18.2% decrease in leaf length. Biomass allocation shifted dramatically from 90.15% leaf allocation in controls to 77.06% under evening supplementation, with stem allocation increasing from 1.92% to 17.33%. SPAD analysis revealed timing-dependent chlorophyll responses, with early enhancement under evening treatment and superior mid-season development under early morning supplementation. These findings establish that 4-hour evening LED supplementation optimally synchronises with Brassica juncea circadian rhythms, providing an energy-efficient enhancement of productivity while highlighting critical morphological consequences for commercial applications in tropical controlled-environment agriculture.