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Because we live in southern Canada, in the late fall, my mother likes to take as much of her garden indoors as she can fit, for the winter.

She usually brings in her potted palms, a few flowers, at least one pot of each of her dozen or so types of herbs, and before moving to a smaller place, she would also bring in as many pots of tomatoes as she could fit.

She arranges them all next to the window, waters them once a week, and plugs one or two lights into a timer that is programmed to turn on just when it’s getting dark, and off just when it’s getting light.

This has always made me wonder if plants can have too much light. I tried Googling it, but all the discussions I found were about burning from too much sun (ie, too much heat/UV/etc.) That is not the issue here since the light(s) are just 13W CFLs. My issue is more with the idea that as far as the plants are concerned, it is always day and night never comes.

When I was studying psychology in University, one thing I found very interesting was the study of sleep and circadian rhythms. I’ll never forget reading about various experiments on sleep deprivation and their effects. Moreover, in my biology courses, we learned how (most) plants have a two-phase cycle: an anaerobic one (i.e. “breathing” CO2) during the day, and an aerobic one (“breathing” O2) at night. These make wonder what effects having a constant source of light would have on the plants. After all; all day and no sleep make humans something something… what about plants?

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I have yet seen no evidence that continuous light is bad for any plant. That might seem a strange thing to say, but no one (including in comments to answers) has mentioned the levels of continuous light considered in the experiments. Of course, really high levels of continuous light aren't going to be good for all plants, but what about low levels? Your CFLs don't sound like high levels of light to me. It sounds plausible that you could safely have them on longer, if not continuously, but this isn't proven either, and I'm not giving opinions one way or the other here. – Shule Jan 24 '15 at 20:46
I have read, recently, that plants DO need regular cycles with light and dark. Lots of stuff happens for plants during the dark or even during dormancy. I'll try finding a decent resource for you. What plants do you know about that get light 24/7? Glad you asked this question! – stormy Apr 1 '15 at 0:00
It should be noted that UV rays are light, and some animals can see the resulting colors. Infrared rays are also light, and they can cause heat, but the heat isn't necessarily the only thing about them that affects plants. Microwaves and radiowaves, etc. are also light, as is visible light. – Shule Jul 20 at 4:07

It would definitely depend on the plant species you are talking about. Some plants specifically thrive in continuous light. For example, in northern Alaska the day-night cycle can during the summer can become 19 hours of light (or more) and 5 hours of night. This allows people to grow giant cabbage (the record weighing approx. 127 lbs).

Some plants respond better than others. In a study they found that 24 hour light increased the growth of chickpeas under the same amount of time as opposed being grown under normal conditions. Roses experience a shorter blossoming period, but more buds were produced and blossomed. Cucumbers and corn experienced no net increase in mass compared to plants grown under normal conditions. Certain plants such as sweet peppers and tomatoes actually experience decreased growth and yields compared to standard conditions and also experienced blistering from continued light.

So truly it does depend on what plant is being grown.

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Unfortunately I was afraid that would be the case. The few discussions I could find mentioned something along those lines. Sadly this answer is likely correct. Sadly because it means a bunch of research. :-( Is there a good resource to find out this kind of information? It would be really frustrating if it requires randomly searching the Internet for each and every kind of plant. (Re: giant cabbage; imagine the size of the kids in that patch!) :-D – Synetech Feb 13 '12 at 19:33
As far as I can tell, plants that are especially sensitive to too much sun (tomatoes, some peppers) I wouldn't grow under continuous light. However if a plant isn't particularly sensitive to too much light (corn, sunflowers, cabbage) they probably would do better, or have no effect to growth in the worst case. Plants are such a diverse group of organisms that you will rarely find anything that they all share in common. – WienerDog Feb 13 '12 at 19:39
Well like I said, sun isn’t really the issue. In winter in Canada, there is sun, but it is not particularly strong, so sun burn isn’t the problem; lack of “sleep” is what I’m wondering about, ie, “burn out ​”. – Synetech Feb 13 '12 at 19:43
Any light, even artificial light gives off heat (infrared). Of course certain lights give off less heat than others. For the tomatoes in the study I don't suspect it was the light itself as much as it was the heat generated by the lamps. – WienerDog Feb 13 '12 at 19:59
I did find this off of Wikipedia which may interest you. "In addition, plants also require both dark and light ("photo"-) periods. Therefore, lights may be turned on or off at set times. The optimum photo/dark period ratio depends on the species and variety of plant, as some prefer long days and short nights and others prefer the opposite or intermediate 'day lengths'." – WienerDog Feb 13 '12 at 20:00

Actually.. Here's a study that shows tomatoes do pretty well in a 24 hour continuous, but they varied the temperature and they didn't get the spotting that had occurred with 24h lighting in the past. Did better than the 16 hour light / 8 hour dark, anyways.

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This is a very, very interesting read - but I point out that this process was done for germination of seedlings - not full blown plants. Although I lost focus part way through the article I believe that it did not look at the effects of 24/7 hour light on fruit setting or "grown up plant" issues. – davidgo Jul 20 at 20:18

Moreover, in my biology courses, we learned how (most) plants have a two-phase cycle: an anaerobic one (i.e. “breathing” CO2) during the day, and an aerobic one (“breathing” O2) at night.

Wrong. There are two main cycles of photosynthesis colloquially called light and dark cycles. The light cycle requires light. The dark cycle does not require dark. It happens constantly because it is independent of light.

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Correct. The dark cycle is also called Calvin cycle – THelper Jun 19 '13 at 8:50
I won't discuss how accurate this is, but this should be a comment, not an answer. Or you could add a few words to what that implies for the OP specifically regarding plants. – Max Feb 28 '14 at 20:07

You need to distinguish between vegging and flowering when it comes to light cycles!

Vegging light cycle can be 18-24 hours under MH (Metal Halide with a good Daylight Spectrum Bulb) or fluorescent lights (T5's are great).

For flowering, cycle 12 hours on, 12 hours off with a good HPS (High Pressure Sodium) bulb.

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I would like to point out that plants do not have an "anaerobic" phase. Plants ALWAYS "breathe" O2. Just the net balance during the day is the production of O2 by photosynthesis because that only occurs under influence of light.

Basically two reactions occur: (Simplified)

reaction one: glucose (C6H12O6) + 6 O2 --> 6 H2O + 6 CO2 (+ heat)

This reaction releases about 2900 kJ/mol (dG ~ -2900 kJ/mol) That energy is used to drive the synthesis of ATP, the "universal energy currency" of all organisms we know of. I will not go further into this as this strays too far off the subject.

reaction two: 6 CO2 + 6 H2O + hv --> glucose (C6H12O6) + 6 O2

In which hv is the energy of a photon of a particular wavelength. I again want to again point out the simplification of this example, as for one glucose is not the only carbohydrate produced and this is just part of a complex system of reactions.

Imagine the cellular respiration (reaction one) occuring at a constant rate, lets say 30 reactions per second (All rate values are arbitrarily chosen). Reaction two is light-dependant so at night the reaction might only occur at a rate of 1 per second due to stray photons. So the net reaction would be 29 times reaction one per second. As the dawn approaches and the plants light exposure increases, so does the rate of reaction two, until it eventually reaches, lets say 100 reactions per second at maximum light exposure. This will mean that at this point the net reaction is 60 reactions per second of reaction two. But the plant still also consumes oxygen (meaning it is NOT anaerobic)! The plant just produces more oxygen in the same time period. If the day and light periods last equally as long the average rate of reaction two would be around 50 reactions per second versus 30 reactions per second of reaction one. This means the overall average reaction rate would be 20 reactions per second. Enabling the plant to store carbohydrates (glucose in this example) and to oxygenate the atmosphere other aerobic organisms require to live.

The requirement of a day-night cycle depends on the particular plant. However all plants do require light at some point, for the reasons above. Also most plants can not grow properly under continuous light due to oxidative stress (accumulation of free radicals) and deregulation of various plant processes (Exhaustion of crucial compounds). I like to imagine it to overdriving an engine, although that damage is irreversible wheras the damage done to plants is partially reversible.

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I grow broccoli sprouts under a fluorescent bulb. I, too, wondered whether continuous light had an adverse effect on germination/growth. My experience has been that this species thrives in 24 hour light conditions. There may be some optimal on/off ratio that would produce even better results, but since the sprouts seem so happy, I don't sweat the details! :)

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