Photosynthesis shuts down at night. The light-dependent reactions that split water molecules and power the conversion of carbon dioxide into sugars cannot run without photons, so when the sun sets, that entire process stops. But plants do not simply go idle in the dark. They switch to a mode that looks, metabolically, more like what animals do around the clock: they consume oxygen, burn stored sugars, and release carbon dioxide. What happens during those dark hours turns out to be just as consequential for a plant’s survival and growth as what happens during the day.
The Switch to Respiration
During daylight, a plant is simultaneously photosynthesizing and respiring, but because photosynthesis runs faster and in the opposite chemical direction, the net effect is that the plant absorbs carbon dioxide and releases oxygen. Once light disappears, only respiration remains. This “dark respiration” takes place in the mitochondria, where glucose molecules are broken down using oxygen, producing carbon dioxide, water, and the energy currency ATP.1PubMed Central. Dark Respiration Measurement from Arabidopsis Shoots The chemistry is essentially the reverse of photosynthesis: instead of building sugar and releasing oxygen, the plant tears sugar apart and consumes oxygen.
This is not a minor housekeeping process. Respiration at night provides the energy plants need for everything from transporting nutrients to assembling proteins to expanding new cells. Without it, a plant would have no way to power its biology during the roughly half of every 24-hour cycle when solar energy is unavailable.
Living Off Savings From the Day
A plant’s nighttime economy depends on carbohydrates stockpiled during the day. Much of this reserve takes the form of starch, which accumulates in leaves while photosynthesis is running. The way plants manage this starch reserve is remarkably precise. In the model plant Arabidopsis, starch degradation at night proceeds at an almost perfectly linear rate, timed so that nearly all the starch is used up just as dawn arrives.2PubMed Central. Circadian control of carbohydrate availability for growth in Arabidopsis plants at night The plant does not burn through its reserves quickly and then starve before sunrise, nor does it hoard too much and waste potential energy.
This careful budgeting is governed by the plant’s internal circadian clock. When researchers manipulated the clock by growing plants in artificially long days (28 hours instead of 24), the starch-degradation rate was thrown off and the plants ran out of carbon before the lights came back on. The resulting carbon starvation visibly reduced growth. The same thing happened in clock-mutant plants grown under normal 24-hour cycles: without proper circadian control, the degradation rate no longer matched the night’s length.2PubMed Central. Circadian control of carbohydrate availability for growth in Arabidopsis plants at night In other words, the clock is not just keeping time. It is actively calculating how fast to spend reserves so the plant stays solvent through the night.3PubMed. Starch and the clock: the dark side of plant productivity
The Burst of CO₂ Right After Sunset
The transition from light to dark is not a clean on-off switch. In the first seconds after a leaf goes dark, there is a brief spike in carbon dioxide release known as the post-illumination CO₂ burst.4PubMed. Measuring and Quantifying Characteristics of the Post-illumination Burst This burst peaks within about eight seconds of the light dropping and lasts only around six seconds before it declines rapidly.5Canadian Journal of Botany. Photorespiration and the postillumination CO2 burst It comes from a process called photorespiration, which is an inefficiency baked into photosynthesis itself. During the day, the enzyme responsible for fixing carbon dioxide sometimes grabs an oxygen molecule instead, creating a byproduct that must be recycled. When the light shuts off, the remnants of that recycling process release one last puff of CO₂.
The size of this burst scales with how bright the light was beforehand and how much oxygen is present. If oxygen is reduced to very low levels, the burst disappears entirely, confirming its link to photorespiration rather than to ordinary dark respiration.5Canadian Journal of Botany. Photorespiration and the postillumination CO2 burst While short-lived, this phenomenon matters to researchers studying carbon balance, because it represents a measurable loss of carbon at every single dusk transition.
Why Plants Actually Grow at Night
One of the more counterintuitive facts about plant biology is that much of a plant’s physical expansion happens after dark. This is especially true for trees. Research tracking radial stem growth found that trees grow mainly at night, with a peak after midnight, when the humidity around their canopy is highest and the air’s drying power is weakest.6PubMed Central. Why trees grow at night During daylight, the atmosphere pulls water out of leaves so aggressively that it creates tension in the plant’s water column, which physically shrinks the stem. Growth during those hours is minimal. Only once the sun goes down and humidity rises can cells take in enough water to expand.
Leaf expansion follows a similar pattern. Respiration fuels the biochemical work of building new cell-wall material, and the carbohydrates available for that work depend on how much starch was stored during the day.7Trends in Plant Science. What Happens to Photosynthesis at Night? So while photosynthesis does the carbon-harvesting during the day, the actual structural payoff, the physical growth, largely happens at night. A plant that cannot properly manage its carbohydrate reserves or that faces unusually warm nights (which increase respiratory costs) will grow less, even if it photosynthesizes normally during the day.
CAM Plants Fix Carbon in the Dark
Most plants cannot do anything resembling photosynthesis without light, but there is a notable group of exceptions. Plants that use Crassulacean Acid Metabolism, known as CAM, have evolved to open their stomata and absorb carbon dioxide at night, when temperatures are cooler and water loss from evaporation is dramatically lower. Cacti, agaves, pineapples, and many succulents use this strategy.
CAM plants store the nighttime CO₂ as malic acid in their cell vacuoles. The carbon source for this acid comes from stored carbohydrates, particularly a large glucan pool that includes starch, which is broken down through glycolysis to supply the necessary building blocks.8Australian Journal of Plant Physiology. The Path of Carbon in CAM Plants at Night During the following day, the stomata close to conserve water, the stored malic acid is released, and the CO₂ freed from it enters the normal light-dependent photosynthetic pathway. The light reactions still need sunlight, but the raw material was gathered in the dark.
This nocturnal acid accumulation is not just a minor tweak on ordinary plant metabolism. A survey across 40 families of vascular plants showed that nighttime acidification is a feature entirely restricted to CAM species. Even though some conventional plants can produce malate during the day, the switch to nighttime malic acid accumulation requires a fundamentally different metabolic setup, one that couples the breakdown of stored carbohydrate to net dark CO₂ fixation.9PubMed Central. CAM photosynthesis: the acid test It is a genuine evolutionary innovation, not a gradual shade of the same process. And despite the roundabout chemistry, the total amount of CO₂ captured over a full 24-hour cycle by CAM plants is comparable to what conventional plants achieve, with the added bonus of far better water efficiency during drought.10PubMed. Achievable productivities of certain CAM plants: basis for high values compared with C(3) and C(4) plants
Stomata Do Not Fully Close at Night
Most people assume that the stomata, the tiny pores on leaf surfaces through which gas exchange occurs, seal shut at night in typical plants, since there is no photosynthesis demanding CO₂ intake. In reality, many species maintain some degree of nighttime stomatal opening, which means they keep losing water to transpiration even in the dark. This seems wasteful: water is leaving the plant without any carbon gain in return.
One explanation that has gained traction is that this nighttime water movement helps plants absorb nutrients. As water flows from soil into roots and upward through the plant, it carries dissolved minerals along with it. Keeping a continuous stream flowing through the night could enhance nutrient delivery to the roots by pulling dissolved minerals toward the root zone through the soil solution.11PubMed Central. Nighttime Stomatal Conductance and Transpiration in C3 and C4 Plants The idea is that the nutritional benefit outweighs the water cost, at least under conditions where water is not scarce. This behavior has been observed in both conventional plant types (C3 and C4), suggesting it is widespread rather than a quirk of a few species.
What Roots Do While Leaves Rest
The activity below ground shifts on a day-night rhythm too. Roots release a cocktail of organic compounds into the surrounding soil, a process called exudation, and this output fluctuates with the time of day. In pea plants, for instance, the amounts of soluble sugars and amino acids exuded into the soil change on a diurnal cycle. When sugar and amino acid pools build up inside root tissues, more of those compounds leak out into the surrounding soil.12PubMed. Root exudation of carbon and nitrogen compounds varies over the day-night cycle in pea These exudates feed soil microbes, which in turn help break down nutrients into forms the plant can reabsorb. The ebb and flow of root exudation means that the microbial community in the soil around roots experiences a rhythmic pulse of food, creating a dynamic underground partnership that mirrors the above-ground light cycle.
Warm Nights Are a Growing Problem
Rising nighttime temperatures are one of the clearest ways that climate change intersects with what plants do after dark. When nights are warmer, respiration speeds up because the underlying chemistry is temperature-sensitive. The trouble is that this faster respiration burns through more of the carbohydrate reserves that were supposed to sustain the plant until dawn, and the extra energy released is often not efficiently captured.
Research on wheat grown under elevated nighttime temperatures found that respiratory CO₂ release increased at the warmer growth temperature, but oxygen consumption actually declined when measured at a standardized temperature. This mismatch suggests the plant’s energy-producing machinery becomes less efficient: more carbon is burned, but less usable energy (ATP) is produced from each molecule of sugar. The result was reduced plant biomass overall.13Journal of Experimental Botany. Wheat respiratory O2 consumption falls with night warming alongside greater respiratory CO2 loss and reduced biomass
In maize, high nighttime temperatures during the grain-filling period led to a roughly 40 percent increase in nighttime respiration rates in some experiments, alongside a significant drop in daytime photosynthetic capacity. Researchers found a strong negative relationship between how much the plant respired at night and how well it photosynthesized the next day, suggesting that heavy nighttime carbon losses carry a hangover effect that impairs the next day’s sugar production.14Journal of Agronomy and Crop Science. High night temperature during maize post‐flowering increases night respiration and reduces photosynthesis, growth and kernel number For agriculture, this is a serious concern. Warmer nights are one of the most consistent features of recent climate trends, and they threaten crop yields through this invisible nighttime carbon drain, even when daytime conditions look fine.
The Cost of Nighttime Biochemistry
Respiration at night does not only fuel growth and maintenance. Plants also carry out energy-intensive biosynthesis after dark, including the production of amino acids, the building blocks of proteins. Because the efficient photosynthetic energy supply is unavailable, nighttime amino acid synthesis has to rely entirely on energy extracted from stored carbohydrates. Modeling work in Arabidopsis shows that the energy cost of making amino acids under nighttime conditions is, in most cases, higher than it would be during the day, even in non-photosynthetic cells.15PubMed Central. Effects of Varying Nitrogen Sources on Amino Acid Synthesis Costs in Arabidopsis thaliana under Different Light and Carbon-Source Conditions In other words, the same biochemical job is more expensive to run in the dark because the plant must rely on its less efficient backup power supply rather than the direct solar energy of photosynthesis.
How Artificial Light at Night Disrupts the System
Urban and suburban plants increasingly face a condition their biology did not evolve for: nights that never get fully dark. Streetlights, building illumination, and other sources of artificial light at night (ALAN) can interfere with the carefully timed processes described above. Studies on deciduous trees exposed to nighttime lighting found disruptions in photosynthetic electron transport, with measurable declines in the efficiency of the systems responsible for moving energy through the photosynthetic apparatus.16PubMed Central. Artificial Light at Night Alters Photosynthetic Electron Transport in Two Deciduous Species
The damage does not just affect the light reactions. Continuous artificial light at night has been shown to inhibit both photosynthesis and respiration, disrupting the carbon balance of leaves during the day. The severity of these effects depends on the intensity of the light and the species involved, but the overall pattern is clear: plants exposed to ALAN may photosynthesize less effectively the following day and have their normal respiratory rhythms thrown off.17Forests. Effects of Artificial Light at Night on Photosynthesis and Respiration of Two Urban Vascular Plants For trees planted along illuminated streets, this chronic disruption could compound over years, potentially affecting long-term health and growth in ways that are just beginning to be studied.
The internal recovery that photosynthetic machinery undergoes during a proper dark period may help explain why uninterrupted darkness matters. After a day of absorbing light energy, plants need to dissipate excess energy and reset certain molecular components. This relaxation process, which involves changes in how the light-harvesting complexes manage excess energy, unfolds over minutes to hours in the dark.18PubMed. Dark recovery of the Chl a fluorescence transient (OJIP) after light adaptation If artificial light prevents or shortens this recovery window, the plant starts the next day with its photosynthetic equipment in a less-than-optimal state.
What Happens Underwater
The nighttime shutdown of photosynthesis is not just a concern for land plants. In ponds, lakes, and aquaculture systems, the algae and aquatic plants that produce oxygen during the day stop doing so at night and begin consuming oxygen through respiration, just like their terrestrial counterparts. The microbial community in bottom sediments also continues to consume oxygen around the clock. The combined effect can cause dissolved oxygen levels to plummet in the hours before dawn, a phenomenon familiar to anyone who manages fish ponds or monitors water quality in shallow lakes. In aquaculture, this nighttime oxygen draw from both phytoplankton respiration and microbial activity in the sediment is a primary reason that aeration systems are often run overnight.
The swing can be dramatic. A shallow pond with heavy algal growth may be supersaturated with oxygen on a sunny afternoon and dangerously low by the early morning hours. Fish kills attributed to “low oxygen” events almost always occur in the pre-dawn window, when the accumulated overnight oxygen consumption has drawn levels below what aquatic animals can tolerate. Understanding that photosynthesis halts in the dark while respiration continues is essential to managing these aquatic systems, whether they are ornamental garden ponds or commercial aquaculture operations.
Early Experiments That Revealed the Day-Night Divide
The realization that plants behave differently in light and dark was one of the landmark discoveries in experimental biology. In the late 1700s, Jan Ingenhousz demonstrated that the green parts of plants purified air (released oxygen) only under the influence of light.19PubMed. Air and the origin of the experimental plant physiology Before his work, the prevailing assumption was that plants improved air quality continuously, day and night. Ingenhousz showed instead that plants in the dark actually “spoiled” air, releasing what we now know as carbon dioxide, just as animals do. This was a conceptual revolution: it established that plants have two fundamentally different metabolic modes, and that light is not just helpful but necessary for the oxygen-producing side of the equation.
Theodore de Saussure refined the picture further, demonstrating that the amount of oxygen released was proportional to the carbon dioxide absorbed, establishing the stoichiometric relationship at the heart of photosynthesis. These early experiments are the reason we understand that nighttime is not simply the absence of photosynthesis but the period when respiration, normally masked during the day, becomes the dominant and visible metabolic activity of the plant.