Nighttime sap flow is the night-time movement of fluid within the sapwood of root, stem, or branch of a plant. There is increasing evidence of occurrence of nighttime sap flow in a range of species, habitats, and ecosystems. In this study, we elucidated the magnitude of nighttime sap flow density and summarized the state of research in this field from the perspective of physiological implications, influencing factors, and the eco-hydrological effects. Generally, nighttime sap flow density is 5%-20% of that at daytime. Nighttime sap flow involves stem refilling and transpiration processes, yet there are only a few studies or techniques to distinguish them. Although the fraction of nighttime sap flow to daytime sap flow is low, the processes of stem refilling and transpiration have significant implications on the growth and survival of plants. The process of stem refilling is beneficial for plants as it increases stem water potential, repairs embolized xylem conduits, and promotes the adaptability of plants to drought stress. Furthermore, the process of transpiration is related to nutrient availability, oxygen transport, and hydraulic lift in plants. The factors controlling nighttime sap flow may have considerable complexity. In addition to the habitat and genetic factors of a plant, the magnitude of nighttime sap flow is mainly affected by meteorological factors. The influence of soil moisture on nighttime sap flow is related to the habitat conditions. Nighttime sap flow also has significant implications on water budgets and hydrological processes at different scales because of water loss at night. Future studies should focus on 1) exploring the relationship between nighttime sap flow and physiological characteristics under different habitat conditions; and 2) quantitatively assessing the effects of nighttime sap flow on hydrological processes, and (3) analyzing the response of nighttime sap flow to global climate change.