Growing plants indoors in the UK gives you far more control than relying on the weather — but one of the most common (and most misunderstood) questions is:
How long should grow lights be on each day?
It’s tempting to assume that more light always equals more growth. In reality, plants need a balanced daily rhythm.
Too little light leads to weak, leggy plants. Too much light — especially without darkness — can cause stress, slow development, and poor long-term results.
This guide explains the ideal grow light schedules for different plant types and growth stages, tailored specifically for UK indoor growing conditions.
If you’re new to indoor lighting or want a broader overview, our Grow Lights UK – Complete Resource Hub brings everything together in one place.
⏱️ Quick Reference: Grow Light Schedules
If you just want a clear answer, use the table below as a reliable starting point for a standard 24-hour cycle:
Crucial rule:
👉 Plants do not benefit from 24/7 light. Darkness is essential.
🇬🇧 Why Grow Light Timing Matters in the UK
In the UK, natural daylight — especially in winter — is often:
- Short
- Weak
- Inconsistent due to cloud cover
Indoors, grow lights aren’t just supplementing sunlight — they are the sun.
That means timing matters just as much as brightness or distance.
🌞 Photosynthesis vs Respiration
Plants don’t simply “switch off” when the lights go out.
- Light period: Photosynthesis (energy creation)
- Dark period: Respiration (energy use, repair, and growth)
Without a dark period, the plant’s internal clock (circadian rhythm) becomes disrupted, leading to stress and reduced performance.
🌱 Preventing Leggy, Weak Growth
If light duration is too short, plants stretch toward the light source, becoming thin and unstable.
Common signs include:
- Leaning seedlings
- Long gaps between leaves
- Weak stems
If this happens, increase daily light hours and check spacing using your Grow Lights Distance Chart UK.
🌱 Grow Light Schedules by Growth Stage
🌿 Seedlings (14–16 Hours)
Seedlings need plenty of light to build strong roots and stems — but they’re delicate.
- Start around 14 hours
- Increase to 16 hours if stretching appears
- Keep light intensity moderate
UK tip: Homes can be cool and drafty in winter. Longer light periods may slightly warm the growing area, but keep an eye on moisture levels so compost doesn’t dry out.
If you’re still early in your setup, it’s worth reviewing How to Set Up Grow Lights before fine-tuning schedules.
🥬 Leafy Greens & Herbs (14–18 Hours)
Lettuce, spinach, basil, coriander, kale, and rocket thrive under long “daylight” conditions.
- 16–18 hours encourages faster harvests
- No flowering stress
- Very forgiving of small mistakes
This category is ideal for beginners using shelving or kitchen setups — especially when paired with advice from Best Grow Lights for Beginners.
🍅 Fruiting Plants (Tomatoes, Chillies, Peppers)
Fruiting crops are energy-hungry and benefit from stable routines.
- 16–18 hours is ideal
- Consistency matters more than maximum hours
- Maintain the same schedule once flowering begins
If you’re using modern LED grow lights, different models produce very different light intensities, which can affect how many hours per day you actually need — something we break down in Top 10 LED Grow Lights UK.
🌿 Low-Light Houseplants (8–12 Hours)
Many common houseplants evolved on shaded forest floors.
Examples include:
- Snake plant
- Pothos
- Peace lily
- ZZ plant
Too much artificial light can stress these plants. Short, gentle light periods usually work best, especially as a winter supplement.
🌙 Do Grow Lights Need a Dark Period?
Yes — always.
Darkness allows plants to:
- Repair cells
- Regulate growth hormones
- Complete respiration
Running grow lights continuously can lead to:
- Burnt or curled leaves
- Nutrient uptake issues
- Slower growth over time
More light isn’t better. Better timing is.
⏲️ The Essential Tool: A Plug-In Timer
Manual switching is one of the most common causes of indoor growing problems.
Plants thrive on predictable routines. Turning lights on at 7am one day and 11am the next creates stress — even if the total hours are similar.
Timer options:
- Mechanical timers: Cheap, reliable, simple
- Digital or smart timers: Precise control and automation
UK energy note:
Using a timer doesn’t just protect plant health — it also helps avoid wasting electricity. With UK energy prices still high, keeping lights off during unnecessary hours (or expensive peak tariffs, if you’re on a smart meter) can noticeably reduce running costs over time.
Timers are especially important in larger setups, such as those discussed in Best Grow Lights for Every Tent Size.
🚫 Common Grow Light Timing Mistakes
- Leaving lights on 24/7
- Constantly changing schedules
- Copying outdoor daylight exactly
- Ignoring seasonal daylight in bright rooms
- “Ignoring the ‘Off’ Period Heat Drop:” In UK winter, if you turn lights off at night in an unheated room, the temperature might drop too low for seedlings. Consider timed heating mats if your ‘dark’ period coincides with a cold UK night.
Pick a schedule and stick with it for at least two weeks before making adjustments.
🔧 How Light Duration Works With Distance & Power
Light timing doesn’t exist in isolation.
If your lights are:
- Very close to the plants
- Particularly powerful
You may need slightly fewer hours per day.
That’s why duration should always be considered alongside spacing — use this guide together with your Grow Lights Distance Chart UK for best results.
✅ Final Thoughts
So — how long should grow lights be on?
Ultimately, the decision of how long should grow lights be on depends on your specific plants, but sticking to the 14–18 hour range is a safe bet for most UK homes.”
- 14–18 hours of light
- 6–10 hours of darkness
- Every day, consistently
Get the timing right and you’ll see:
- Stronger growth
- Healthier plants
- Fewer problems overall
Once you’ve mastered light schedules, the next step is understanding what type of light actually matters — which we’ll cover next in a guide comparing full spectrum vs red and blue grow lights.
For those interested in how artificial lighting is used more broadly beyond food crops, the RHS also has a helpful overview of using artificial light for houseplants.
📎 Related Articles
❓ Grow Light Timing: Frequently Asked Questions
Can I leave my grow lights on 24 hours a day to speed up growth?
No. While it might seem logical, plants need a dark period to complete their daily biological cycle. During darkness, plants stop creating energy (photosynthesis) and start using it to build cells and repair themselves (respiration). Running lights 24/7 can lead to stunted growth, leaf burn, and nutrient deficiencies.
How do I know if my plants are getting too much light?
Watch the leaves. If they are curling downwards, turning yellow/brown at the tips, or looking “bleached” while the rest of the plant is green, they are likely getting too much light. This is often a combination of the lights being on too long and being positioned too close to the canopy.
Will grow lights significantly increase my UK electricity bill?
Modern LED grow lights are very efficient. For example, running a 100W LED for 16 hours a day typically costs around 30p–40p per day (depending on your specific UK tariff). Using a plug-in timer ensures you aren’t wasting money by accidentally leaving lights on overnight.
Should I change the light schedule when my plants start flowering?
For “photoperiod” plants (like certain exotic flowers), yes. However, for common UK indoor crops like tomatoes and chillies, you should maintain a consistent 16–18 hour schedule throughout their life. Consistency is more important for these plants than mimicking the declining light of autumn.
Can I run my grow lights at night to save on electricity?
Yes, but only if you can keep the plants in total darkness during the day. Many UK growers run lights at night to take advantage of cheaper “off-peak” energy tariffs (like Economy 7) or to keep the grow area warm. However, for this to work, your plants must have a “blackout” period during the day. If they are on a windowsill or in a bright room, the natural daylight will prevent them from resting, leading to stress. This “night-shift” method works best in grow tents or rooms with thick blackout curtains.
Can I use a regular lamp instead of a grow light for the same amount of time?
Regular household bulbs (even LEDs) don’t provide the specific “Full Spectrum” light (PAR) that plants need to grow. While you can leave a desk lamp on for 16 hours, the plant won’t be able to photosynthesise effectively and will likely become leggy and weak.
My seedlings are leaning to one side; should I leave the light on longer?
Leaning usually means the light isn’t intense enough or is too far away, rather than the duration being too short. Ensure your light is directly overhead and check your Grow Lights Distance Chart UK to see if it needs to be lowered before you decide to increase the “on” time.
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