they grow faster at night
During elongation, height grew faster at night in 4 of 5 boxes; the 95% interval excludes zero in 2 of them (box 3: by 0.21 px/h, 0.12–0.33). Night also differs in temperature, humidity and morning condensation, so darkness cannot be separated yet. No such measurement turned up in the available literature.
own data · 5 boxes