Changes in the photoperiod (defined as the period of time each day during which your plants receive light, or day length) can result in a photoperiod stress syndrome, which can induce stress response genes in the plant. Plants have and follow a circadian rhythm (as do you and I,) which is a natural oscillation that repeats roughly every 24 hours. Our bodies and systems try to predict things based on past things. You set your alarm for 5:30 am every day on your phone, and every day you wake up at 5:30. Say you forget to set your alarm on your phone one night, but you wake up at 5:30 am anyway. Your body trains itself and so do plants.
From an article on the site Frontiers in Plant Science: "Plants use the circadian clock to anticipate diurnal changes. Circadian rhythm predicts a 24-h cycle with 16 h of light and 8 h of darkness in response to abiotic and biotic factors as well as the appropriate temperature."
So when you're changing up that rhythm outside of that general cycle, it's potentially stressing them out.
As to outdoor light, the amount of light intensity drastically drops as the sun starts to go down, but brightness and intensity are not the same things. Moonlight at full moon equals only 1/200,000th sunlight at noon.
Lumen is a measurement of the human eyes ability to perceive the brightness emitted by a light source. Intensity is the power per unit area; it is a physical quantity. Brightness involves how the human visual system perceives light, and it is not a physical quantity.
So while you can read a book on a full moon night because it's really bright, it's not really intense.
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