Plants grow and transpire water during the day and night. Recent work highlights the idea that night-time transpirational water loss is a consequence of allowing respiratory CO2 to escape at sufficiently high rates through stomata. Respiration fuels night-time leaf expansion and requires carbohydrates produced during the day. As carbohydrate availability and growth are under the control of the plants’ internal clock, so is night-time transpiration. The cost of night-time transpiration is that water is lost without carbon being gained, the benefit is a higher efficiency of taken up water for use in leaf expansion. This could provide a stress acclimation process. Plants loose water at significant rates during the night through ‘night-time transpiration’. Night-time transpirational water loss is most likely the consequence of having respiratory CO2 escape at sufficiently high rates through stomata. As night-time respiration fuels growth and depends on daytime provision of storage carbohydrates, there must exist some regulation, involving the internal clock, between daytime photosynthesis and growth, and night-time transpiration and growth. Night-time growth presents a more efficient use of taken up water for leaf cell expansive growth compared with daytime growth, and could represent an overlooked stress acclimation process.