Growing plants indoors gives you much more control over light than relying on a UK windowsill, but that creates another question:
How long should grow lights be on each day?
For many seedlings, herbs and indoor food crops, around 14–16 hours of light per day is a sensible starting point. Some plants can benefit from longer days, while houseplants and plants receiving plenty of natural daylight may need considerably less artificial light.
The important point is that hours alone don’t determine whether a plant is receiving enough light. A weak grow light running for 16 hours can deliver less useful light than a powerful LED running for 12 or 14 hours.
This guide explains how to choose a practical grow-light schedule, what to change if your plants aren’t responding well, and how light duration works alongside intensity and distance.
For a broader introduction to choosing and using grow lights, see my Grow Lights UK complete guide.
Quick Reference: Grow Light Schedules
These timings are useful starting points when a grow light is providing most of the plant’s light. Natural daylight, light intensity and distance from the plant can all change how many hours are actually useful.
These figures assume a normal 24-hour cycle. A powerful LED can deliver considerably more useful light in 14 hours than a small bulb can in 18, so don’t treat the timer setting as the only measure of whether a plant is getting enough light.
Why Grow Light Timing Matters in the UK
Artificial lighting is especially useful through the UK autumn and winter because natural days are shorter and sunlight is weaker.
When grow lights provide most of a plant’s useful light, duration matters alongside intensity and distance.
A plant under a weak light may benefit from a longer daily schedule, while a plant under a much stronger LED may receive enough useful light in fewer hours.
That means three things need to work together:
- How powerful the light is
- How far it is from the plant
- How many hours it runs
This is why simply leaving a grow light on for longer is not always the best way to improve growth.
Photosynthesis vs Respiration
Plants don’t simply “switch off” when the lights go out.
Photosynthesis requires light, but respiration happens both during the day and at night. Plants also use the change between light and darkness as part of their internal daily rhythm.
That means a regular light/dark cycle is generally better than simply leaving grow lights on continuously.
For most home growing, the practical takeaway is simple: give plants a consistent daily schedule rather than trying to maximise light hours at all costs.
Preventing Leggy, Weak Growth
If seedlings become tall, thin or start leaning, the problem is often insufficient light intensity rather than simply too few hours.
Common signs include:
- Long gaps between leaves
- Thin, weak stems
- Seedlings leaning towards the light
- Pale or stretched new growth
Before increasing the daily light period, check that the grow light is powerful enough, positioned directly above the plants and close enough to provide useful intensity.
You can use my Grow Lights Distance Chart UK to check whether the light needs to be moved closer.
Grow Light Schedules by Growth Stage
Seedlings (14–16 Hours)
Seedlings generally need plenty of light because insufficient intensity encourages them to stretch.
A good starting point is 14–16 hours per day. If seedlings become tall and spindly, don’t automatically add more hours. First check whether the light is:
- Powerful enough
- Positioned directly above the plants
- Close enough to provide useful intensity
UK winter temperatures are another consideration. Grow lights can add a little warmth while they are running, so an unheated room may cool noticeably once they switch off. If seedlings need more warmth overnight, a thermostatically controlled heat mat is usually a better solution than simply extending the light period.
If you’re still setting everything up, see How to Set Up Grow Lights Indoors.
Leafy Greens & Herbs
Lettuce, spinach, basil, coriander, kale and rocket are all well suited to indoor growing under LEDs.
For most setups, around 14–16 hours of light per day is a sensible starting point.
These crops are fairly forgiving, but the same rule still applies: a longer schedule does not automatically compensate for a weak or badly positioned light. If growth is slow or stretched, check intensity and distance before increasing the number of hours.
This type of crop is particularly well suited to shelving or smaller kitchen setups, especially with lower-powered lights designed to cover a compact area.
If you’re choosing a simple setup, see my Best Beginner Grow Lights post..
Fruiting Plants — Tomatoes, Chillies and Peppers
Tomatoes, chillies and peppers need more light overall than most leafy greens and houseplants, but that does not necessarily mean they need the longest possible daily schedule.
For most home setups, around 14–16 hours per day is a sensible starting point.
What matters just as much is whether the light is strong enough. A powerful LED running for 14 hours may deliver more useful light than a weak lamp running for 18.
Once plants are established and flowering, consistency is usually more useful than repeatedly changing the timer.
If growth is weak, pale or stretched, check the light’s power and distance before simply increasing the number of hours.
If you’re deciding whether your current light is suitable for fruiting crops, see my Best LED Grow Lights UK comparison.
Low-Light Houseplants
Low-light houseplants usually need much less artificial light than seedlings, herbs or fruiting crops.
For plants such as snake plants, pothos, peace lilies and ZZ plants, around 8–12 hours of artificial light can be a sensible starting point.
The key difference is that houseplants often receive useful natural daylight as well. A plant near a window may already be getting several hours of light before the grow lamp switches on.
So rather than automatically using a long 14–16 hour schedule, take into account:
- How bright the room is
- How close the plant is to a window
- The season
- The strength of the grow light
For many houseplants, shorter supplemental lighting is more appropriate than treating the grow light as their only light source.
Do Grow Lights Need a Dark Period?
For most home growers, yes — a regular dark period is the sensible approach.
Plants use light for photosynthesis, but their internal biological rhythms also respond to the change between light and darkness. Different species tolerate long photoperiods differently, so simply leaving grow lights on continuously is not a reliable way to improve growth.
For common indoor crops such as seedlings, herbs, tomatoes and chillies, a consistent daily light/dark cycle is simpler, cheaper and easier to manage than 24-hour lighting.
The exact dark period does not need to be treated as a rigid rule, but in most home setups there is little benefit in trying to eliminate darkness altogether.
The Essential Tool: A Plug-In Timer
A timer is one of the simplest ways to make a grow-light setup more consistent.
Mechanical timers are cheap and reliable, while digital or smart plugs give you more control over switching times. I prefer a smart plug because I can set the schedule once and also monitor how much electricity the light is actually using.
I use the Tapo P110 for my own grow lights. I normally set the schedule once and leave it alone rather than changing the hours from day to day, and the energy monitoring also lets me see the real power use and running cost. I cover it in more detail in my Tapo P110 Smart Plug Review.
A timer also helps prevent wasted electricity from lights being left on longer than intended.
If you are on a time-of-use tariff with genuinely cheaper off-peak electricity, you may also be able to shift some of the lighting period into cheaper hours — as long as the plants still get a sensible and consistent daily light/dark cycle.
Common Grow Light Timing Mistakes
Leaving lights on continuously
More hours do not automatically mean more growth. For most home setups, a normal daily light/dark cycle is a better starting point than running lights 24/7.
Constantly changing the schedule
Plants respond better to a stable routine than frequent adjustments. Unless there is an obvious problem, keep the same schedule for at least a couple of weeks before changing it.
Trying to fix weak light with extra hours
If seedlings are stretching or growth is weak, the problem may be intensity or distance rather than duration. Moving the light closer or using a stronger light may help more than extending the timer.
Ignoring natural daylight
Plants near windows are already receiving some light before the grow lamp comes on. Artificial-light hours should be considered alongside that natural daylight.
Forgetting about the temperature drop
Grow lights produce some heat while they are running. In an unheated UK room, shed or garage, temperatures can fall noticeably once the lights switch off.
If tender seedlings need more warmth during the dark period, a thermostatically controlled heat mat is usually a better solution than leaving the grow light on longer.
How Light Duration Works With Distance & Power
Light duration does not work in isolation.
A plant under a powerful LED receives much more useful light each hour than the same plant under a small, low-powered bulb. Distance matters too, because moving a light closer usually increases the intensity reaching the leaves.
That means:
- A weaker light may need to run for longer
- A stronger light may provide enough light in fewer hours
- Moving a light closer can sometimes help more than extending the timer
This is why professional growers often think in terms of Daily Light Integral (DLI) — the total amount of photosynthetically useful light a plant receives over the course of a day.
You do not need to calculate DLI for a simple home setup, but the principle is useful:
Light intensity × time = total daily light
So if a plant is already receiving plenty of intensity, leaving the light on for extra hours is not automatically an improvement.
For more on the distance side of this, see my Grow Lights Distance Chart UK.
Final Thoughts
For most home growers, around 14–16 hours of light per day is a sensible starting point for seedlings, herbs and many food crops.
The exact number matters less than the overall balance between:
- Light intensity
- Distance from the plant
- Daily running time
- Any natural daylight the plant already receives
If growth is weak or leggy, check intensity and positioning before simply adding more hours.
Above all, keep the schedule consistent. Put the light on a timer, choose a sensible starting point and give the plants enough time to show whether the setup is working.
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 Guides
- Grow Lights UK
A complete overview of grow light types, setup basics, and how indoor lighting is used in UK homes. - How to Set Up Grow Lights Indoors
Step-by-step guidance on positioning lights, using timers, managing airflow, and avoiding common setup mistakes. - Grow Lights Distance Chart UK
Clear guidance on hanging height and spacing at different growth stages to prevent light stress. - Grow Light Wattage Explained
Explains why “1000W” LED claims are misleading and how real power draw affects intensity, heat, and running costs. - Best Indoor Grow Lights UK for Beginners
Simple, efficient LED grow lights that are easy to run on consistent daily schedules in typical UK homes.
Grow Light Timing: Frequently Asked Questions
Can I leave my grow lights on 24 hours a day?
For most home setups, I wouldn’t recommend it. Some plants tolerate very long light periods better than others, but continuous lighting does not automatically mean faster or healthier growth.
A consistent daily light and dark cycle is simpler, uses less electricity and suits most seedlings, herbs, tomatoes and chillies.
How do I know if my plants are getting too much light?
Possible signs include pale or bleached leaves, scorched-looking patches, curling, or unusually compact growth.
However, this is often an intensity or distance issue rather than simply a problem with the number of hours. If you are using a powerful LED, try checking the hanging height before drastically shortening the daily schedule.
Will grow lights significantly increase my UK electricity bill?
It depends mainly on the actual power draw of the light and how many hours you run it.
Using an electricity rate of 26.11p/kWh, a genuine 100W grow light running for 16 hours per day uses 1.6kWh and costs approximately 42p per day, or around £12.53 over 30 days.
Your actual cost will depend on your tariff, and lower-powered lights will cost proportionally less.
Should I change the light schedule when tomatoes or chillies start flowering?
Usually, there is no need for a dramatic change. Tomatoes and chillies do not normally need a shorter day to trigger flowering in the way some strongly photoperiod-sensitive plants do.
Maintaining enough light intensity and keeping the schedule consistent is generally more important than changing the timer once flowers appear.
Can I run my grow lights at night to save on electricity?
Yes, particularly if you have a tariff with genuinely cheaper overnight electricity.
Just remember that natural daylight also contributes to the plant’s total light period. If the grow light runs all night and the plant then sits beside a bright window all day, the effective light period may become much longer than intended.
Can I use a regular LED bulb instead of a grow light?
Potentially, especially for seedlings or very small plants. Ordinary white LED bulbs produce wavelengths that plants can use for photosynthesis.
The main limitation is usually intensity and coverage. Household bulbs are designed to light a room, not to deliver a strong and even concentration of light onto plants. Purpose-built grow lights are therefore normally much more practical for larger plants or faster growth.
My seedlings are leaning to one side. Should I leave the light on longer?
Probably not as your first response.
Leaning or stretching usually suggests that the light is too weak, too far away, or coming mainly from one direction. Position the light directly above the seedlings and check its distance before increasing the number of hours.
See my Grow Lights Distance Chart UK for more guidance on positioning.