<p>Chlorophyll fluorescence is a commonly used indicator of photosynthetic capacity. Crassulacean acid metabolism (CAM) plants absorb CO₂ during the night, but nighttime gas exchange alone does not readily represent CAM. This study was conducted to determine how light levels, daytime electron transport rate (ETR), and nighttime CO₂ uptake are related. <i>Phalaenopsis</i> Queen Beer ‘Mantefon’ clones were treated with a combination of three light intensity levels of 50, 100, and 200 µmol∙m^-2∙s^-1 and three daylengths: 8&#xa0;h, 12&#xa0;h, 16&#xa0;h, resulting in daily light integrals (DLI) between 1.44 and 11.52&#xa0;mol∙m^-2∙d^-1. Daytime ETR linearly increased with increasing DLI levels; however, nighttime CO₂ uptake reached saturation point around 6&#xa0;mol∙m^-2∙d^-1, and then decreased. As a result, daytime ETR and nighttime CO₂ uptake were linearly correlated within 1.44 to 5.76&#xa0;mol∙m^-2∙d^-1 DLI levels in this study. Under 12&#xa0;h of daylength, despite the higher DLI level and ETR values, the amounts of nighttime malic acid accumulation in 200 µmol∙m^-2∙s^-1 treatment were similar to those in 100 µmol∙m^-2∙s^-1 treatment. On the following day, when all plants were exposed to uniform light conditions of 100 µmol∙m^-2∙s^-1, they exhibited identical ETR values, despite significant differences in CO₂ uptake rates observed in some plants the previous night. These results indicated that the daytime DLI level is a major factor for nighttime CO₂ uptake, but DLI levels over a specific range have minimal effects on the carbon-gaining of CAM orchid <i>Phalaenopsis</i>.</p>