Grow Light vs Sunlight: An Evidence-Based Comparison

In this guide, we’ll analyze how qualities like adjustability, intensity, and spectrum differ between sunlight and various grow light systems.

We’ll explore the impacts on yields, flavor, blooms, and more.

By the end, you’ll understand the tradeoffs to make the optimal lighting decision for your indoor garden goals and environment.

Quick Key Takeaways

Spectrum: Sunlight has a full spectrum that plants evolved under. But most grow lights emit portions of this spectrum, with full-spectrum LEDs coming closest to natural light quality and balance.
Intensity: Direct overhead tropical sunlight peaks at ~100,000 lux but averages ~10,000 lux even in cloudy regions. Grow lights typically provide between 5,000-50,000 lux depending on the technology.
Consistency: Sunlight varies in duration and intensity based on weather, seasons, time of day, etc. Grow lights provide automated precision timing and stable intensity unaffected by external factors.
Adjustability: Grow lights offer exceptional control over photoperiod, intensity, spectrum, and placement to optimize the light recipe for each growth stage. Natural sunlight lacks such customization capabilities.
Impacts on Growth: Sunlight produces the most natural, highest-yielding results thanks to intensity and full spectrum access. But grow lights allow extending and calibrating lighting to facilitate potentially faster growth cycles in some cases.
Costs: Sunlight itself has no operating expenses, but supporting infrastructures do. LED grow lights have higher upfront costs but lower lifetime expenses from electricity savings and longevity vs alternatives.
Recommendation: Leverage abundant sunlight when available, choosing the right grow lights to fill gaps in intensity, duration, or spectrum based on the environment, seasons, and plants’ specific needs for balanced results.

How Sunlight Affects Plant Growth

Sunlight provides the light energy that enables photosynthesis, fueling plant growth and development.

But its impact depends on key qualities like spectrum, intensity, consistency, and direction.

Enabling Photosynthesis: Photosynthesis uses sunlight to convert carbon dioxide and water into chemical energy the plant uses for food and growth.
Spectrum Effects: Sunlight contains a full spectrum of light, but plants don’t use all wavelengths equally in photosynthesis. Blue and red wavelengths have the greatest effects, while green is mostly reflected and unused.
Light Consistency: Outdoor plants depend on the consistent daily patterns of sunlight. But fluctuations between sun and shade reduce photosynthetic efficiency and acclimation capacity. Some fluctuations are normal and plants have mechanisms to deal with them.
Intensity Impacts: Higher-intensity sunlight results in faster photosynthesis and growth, up to a point. Excessively high intensity can damage plants.
Photoperiod Effects: Day length triggers blooming in photoperiodic plants. Variation from natural cycles disrupts growth.

In essence, sunlight enables photosynthesis and fuels growth. However, factors like intensity, spectrum consistency, and direction impact the results.

Key Features and Capabilities of Grow Lights

Grow lights have revolutionized indoor gardening, offering a range of features that make them an excellent alternative or supplement to natural sunlight.

Here are some key features and capabilities of grow lights:

Adjustability: Grow lights offer a high degree of adjustability, allowing you to customize the proximity, angles, and height to cater to your plant’s needs.
Intensity and Spectrum: Grow lights can match or even boost the intensity of sunlight, providing a full, customized spectrum. They emit specific light spectrums, both red and blue lights, essential for different stages of plant growth.
Automated, Consistent Exposure: Unlike sunlight, which varies in intensity and duration throughout the day and year, grow lights offer automated, consistent exposure.
Supplemental Lighting Capabilities: Grow lights can provide supplemental lighting, especially in indoor settings where natural sunlight may be limited or insufficient.
Automation Capabilities: Modern grow lights come with advanced automation capabilities, allowing for the adjustment of various variables over time. This includes the ability to vary the color temperatures and spectral outputs from the grow light, imitating outdoor conditions.

Grow lights offer a range of features and capabilities that make them a valuable tool for indoor gardening.

Whether used as a supplement to natural sunlight or as the primary light source, grow lights can significantly enhance plant growth and productivity.

With that said, let’s compare sunlight vs. growing light based on various important factors.

A. Location Limitations

Sunlight Variability by Geography and Climate

Sunlight availability varies significantly by geography, climate, and seasons. At higher latitudes, there are shorter winter day lengths and fewer total daylight hours annually (source).

For example, in Portland, Maine (43°N latitude), daylight is under 10 hours from November to early February each year. This limits winter plant growth. Many plants require over 10 hours of daylight for buds and flowers to develop properly.

Even in equatorial regions with abundant sunlight, the duration is still limited to ~10-12 hours per day.

The intensity also varies throughout the day based on the sun’s position. UV radiation intensity is highest at midday between 10 am and 4 pm when the sun is most direct.

It also varies by latitude and altitude—there is more intense UV closer to the equator and at higher elevations.

Sunlight can be reduced on cloudy days.

Thick clouds block more UV radiation, but thin clouds provide little protection. Reflective surfaces like snow, water, and sand also intensify UV exposure (source).

Consistent and Controllable Grow Lights

Grow lights allow full control over light duration, intensity, height/direction, and spectrum to suit each plant’s needs. They enable optimized ~12-16 hour photoperiods for maximum growth vs. sunlight’s 10–12 hours.

Indoor facilities with grow lights solve issues around inadequate outdoor sunlight. They offer year-round consistency, not limited by seasons, weather, or geography.

For example, vertical farming with grow lights is gaining popularity in dense urban areas where outdoor space is limited. It enables local fresh produce without rural land or long transport.

However, the intensity of current grow lights is lower than direct equatorial midday sunlight. High-powered LED and HID grow lights can match or exceed lower-intensity sunlight in some cases.

KEY TAKE HOME: Sunlight provides the most powerful and natural light for plant growth, but varies by climate and season. Grow lights enable precise control over light quality, consistency, and daily duration to optimize plant development in any location.

B. Spectrums and Intensity

When it comes to providing light for plant growth, major differences arise between sunlight and artificial grow lights regarding the spectrum properties and intensity levels.

Sunlight delivers the entire spectrum of light, from ultraviolet (UV) to visible and infrared, that plants have evolved under.

In contrast, most grow lights emit only certain portions of this spectrum.

However, full-spectrum LED grow lights attempt to mimic the natural balance of sunlight quite closely.

Spectrum Differences

Sunlight provides a continuous distribution of wavelengths ranging from 300 nm in the UV to over 1000 nm in infrared (source).

Importantly, it contains a balanced mixture of blue, green, and red light within the visible range from 400-700 nm, all of which drive photosynthesis.

Grow lights like LEDs emit light concentrated into narrower wavelength peaks rather than a continuous spectrum.

Full spectrum LED models aim for a blue, green, and red balance similar to sunlight, while other grow light types emphasize certain colors over others.

For example, high-pressure sodium (HPS) lights are dominated by yellows and reds.

IMPORTANT: Most grow lights lack UV light, which aids in plant health and pest resistance.

Intensity Differences

When it comes to intensity, direct overhead tropical sunlight can reach up to 100,000 lux, with 10,000 lux on average days even in cloudier regions (source).

This far exceeds any artificial grow lights, which typically provide between 5,000-50,000 lux depending on the lighting technology used (source).

SIDENOTE: Some high-power LED lights can approach 100,000 lux.

However, since grow lights hang much closer to the canopy than the sun, they can match or sometimes exceed the intensity of weaker indirect sunlight.

New high-power LED fixtures can also approach the intensity of direct sunlight.

But in general, sunlight has a much higher maximum intensity, while grow lights offer a more controlled range appropriate for healthy plant growth.

C. Consistency and adjustability

Consistency and adjustability are two areas where grow lights really shine over natural sunlight.

They enable finer calibration and control over key factors that determine successful plant growth and development.

Grow Lights Provide Consistent, Optimized Lighting

As the research data shows, grow lights offer exceptional consistency in duration, intensity, height from plants, and other variables:

They provide automated lighting with precise timings unaffected by external factors like weather or seasons. Common exact photoperiods (light duration) are 16-18 hours for vegetative growth and 12 hours for flowering (source).
Light intensity in terms of PPFD (photosynthetic photon flux density) does not fluctuate either. It stays constant based on the settings, allowing optimization for each growth stage.

So in essence, grow lights enable superior consistency and control over lighting. This allows “dialing in” the ideal light recipe to facilitate healthy growth.

Natural Sunlight Fluctuations Are More Difficult to Manage

In contrast to such stability with grow lights, sunlight naturally varies in both duration and intensity:

The duration and intensity fluctuate day-to-day based on the time of day (the highest intensity is midday), weather conditions, seasons, latitude, etc.
For instance, winter daylight may last under 10 hours in northern regions, which limits growth for many plants needing over 10-hour photoperiods.

So these sunlight variations make it more challenging to provide stable conditions optimized for plant development. Supplemental lighting helps overcome this issue.

Adjustability Is A Major Advantage of Grow Lights

Beyond just consistency, grow lights offer exceptional adjustability:

Photoperiod can be easily set to any duration from 0 to 24 hours as the plants require.
The light intensity can be precisely controlled and gradually adjusted up or down (e.g. 30,000 to 90,000 lux) by changing the light’s height, number, or output power.
The emitted light spectrum can also be customized by using specific LED colors, combinations, etc. This allows tailoring it to the right spectrum for each growth stage.

Such precise adjustability allows for optimizing the light as plants grow. This degree of control is impossible to achieve with outdoor sunlight (source).

D. Impacts on yield, flavor, and nutrition

When analyzing important factors like yield, flavor profiles, and nutritional content, some key differences arise between sunlight and various grow light systems.

Yield Differences Are Highly Variable

Regarding yields, research shows sunlight typically enables higher production thanks to its sheer intensity.

However, grow lights allow extending photoperiods to 24 hours, enabling faster grow cycles and potentially more harvests per year.

For example, with tomatoes grown indoors under LED inter-lighting, the light recipe and plant spacing optimization increased the yield by 17-50%, compared to controls without supplemental lighting.

Flavor and Aroma Production

Flavors and aromas depend on the complex biosynthesis of plant secondary metabolites like terpenes.

Sunlight provides the full spectrum that plants have evolved under, yielding balanced flavor profiles.

Though evidence is limited, some studies show that grow lights can skew flavor balances in some species.

For example, combinations of red and blue LED lighting decreased aroma production in cucumbers by about 50% compared to white fluorescent lighting.

Another study found that narrowband red LED lighting altered aroma volatile concentrations in tomatoes compared to white LEDs, indicating light spectrum can impact aromas.

Still, effects likely depend on the specific wavelengths blended in grow light recipes.

Nutrient Levels and Antioxidants

Regarding nutrition, sunlight leads to higher levels of some compounds like vitamin D precursors. However, vitamin D requires UVB wavelengths that most grow lights lack (source).

However, research consistently shows that properly designed grow light blends increase antioxidants, vitamins, minerals, and other beneficial nutrients.

In essence, both natural and artificial light sources have tradeoffs for yield, flavor, and nutrition. Sunlight produces the most natural results — grow lights enable optimization.

E. UV exposure

UV light, specifically UV-B and UV-C wavelengths in small doses, can benefit plants by increasing the production of protective secondary metabolites like flavonoids and enhancing disease resistance pathways (source).

However, excessive UV causes damage, so supplemental UV must be applied judiciously.

Comparison of sunlight vs grow lights UV levels

Outdoor sunlight contains much higher levels of UV radiation than any indoor grow light.

For example, summer sunlight may provide 10-30 watts per meter squared of total UV radiation at midday, including 5-15 W/m^2 of UV-B (source).

High-quality LED grow lights emit less than 1 watt per meter squared of UVB radiation, with levels around 0.2 W/m^2. They predominantly produce UVA wavelengths, with very little UVB emission.

HID lights like high-pressure sodium lamps emit some UV radiation, but it is mostly filtered out by the protective glass casing around the bulb (source).

T5 fluorescent tubes and ceramic metal halide (CMH) lights also naturally emit some UV light including UVB wavelengths, in varying amounts.

To sum it all off: at its peak, direct summer sunlight can have over 10 times the total UV intensity of typical indoor grow lighting, but winter sunlight can be less intense (source).

FURTHER READING: T5 vs T8 Grow Lights: Which Is Better for Your Indoor Farm

Research on supplemental UV effects

Here are a few research examples of using supplemental UV for growing plants:

Tomato seedlings under LEDs + UV-B diodes had 2-fold higher beneficial flavonoids vs LEDs alone. This shows that UV stimulates protective compounds (source).
Lettuce grown under LEDs + UV-A was more compact with increased antioxidants compared to LEDs alone (source).
UV-A for microgreens increased concentrations of health-promoting phenolics and antioxidants (source).

These studies show the practical benefits of UV in enhancing the quality and growth of indoor edible crops.

Recommendations for using UV light

Based on research, here are some tips for safely using supplemental UV light:

Start with low intensities (2-3W per square foot) and increase gradually while monitoring plant health.
Limit daily exposure to 2 hours split into 1-hour sessions.
Position UV lights 12-24 inches above plants to prevent damage.
Pair UV-A/UV-B lights with other full-spectrum grow lights for balanced exposure.

When used properly, UV light can significantly improve plant growth, yield, and quality.

F. Operating Costs

When weighing sunlight against grow lights, operating costs become a major consideration

Electricity is the predominant ongoing expense for most grow lighting setups. Meanwhile, sunlight itself comes free of charge besides infrastructure like greenhouses.

Equipment costs: LED fixtures are expensive upfront, from $500 to $1500 each at commercial scales depending on power. Replacing HPS/fluorescent systems costs less but burns more electricity.
Maintenance and bulb replacement: LEDs last up to 5-10 years (25x longer than incandescent) before whole fixture replacement is needed. HPS bulbs need changing every 1-2 years, costing $100+ each time.
Cooling requirements: Grow lights produce large amounts of heat. High-powered AC and dehumidification represent up to 50% of a cultivation facility’s energy use. Sunlight does not add thermal load directly. Open-air greenhouses utilize passive ventilation.

In total, while LED fixtures have higher upfront expenses, their energy efficiency and longevity lead to much lower lifetime costs.

KEY TAKE HOME: By combining sunlight with strategic LED supplemental lighting, overall costs can be optimized for sustainable results.

Optimizing a Combined Solution

Using both natural sunlight and supplemental grow lighting together can be an effective strategy for plant growth.

Sunlight provides the full spectrum of light that plants need, but supplementing with grow lights allows you to provide consistent lighting when sunlight is limited by weather, seasons, or shading.

The key is using sunlight whenever abundantly available, and filling the gaps with grow lights tailored to the needs of your plants.

With some planning around placement and adjusting variables like duration and intensity, you can support healthy plant growth year-round.

Adjusting Variables for Growth Stages and Seasons

In peak summer months, rely more on abundant sunlight, using grow lights to extend day length if needed.
In winter months when sunlight is limited, use more supplemental lighting to provide sufficient intensity and hours of light per day.
As plants mature and transition growth stages, adjust durations and intensities. Seedlings need 14-16 hour days while flowering plants need at least 12 hours.

Adequately adjusting these variables ensures plants have the optimal light recipes through each phase of development.

Use Cases and Examples

Here are some examples of effectively using sunlight and grow lights together:

Greenhouses in cold climates rely heavily on sunlight in summer months, opening vents and roof panels to maximize exposure. In winter, supplemental HID lighting provides intensity and day length extension (source).
Indoor growing spaces near windows use sunlight when available as the primary light source, with LED growing lights filling in gaps to ensure 12-16 hour photoperiods based on the growth phase (source).
Outdoor gardens can use LED grow lights on adjustable mounts to provide extra hours of sunlight before dawn or after dusk, especially for fruiting plants that benefit from 14+ hour days.

KEY TAKE HOME: Leverage sunlight whenever abundantly available. Choose grow lights to fill gaps in intensity, hours of light, or spectrum. And adjust placements and variables based on plant and season.

Conclusion

After thoroughly analyzing differences in spectrum, intensity, consistency, costs, and other factors, the evidence clearly shows that a combined solution of leveraging both sunlight and supplemental grow lights is optimal for indoor plant growth.

This integrated approach takes advantage of sunlight’s natural full spectrum while using grow lights to fill gaps and inconsistencies in winter months or indoor settings.

Grow lights also enable extending photoperiods and calibrating variables to each plant’s needs.

In the end, here are the key takeaways for you:

Leverage sunlight whenever abundantly available.
Choose the RIGHT grow lights to fill gaps in intensity, hours of light, or spectrum.
Adjust placements and variables based on plants and seasons.

Follow these guiding principles of using both natural and supplemental lighting to set up your plants for sustainable success.

Meet your guide
Dhruvir Zala

I’m the guy behind Farmingram, where I nerd out on all things indoor agriculture. Through rigorous research, my goal is to get more people excited about where agriculture is headed.