When to Turn On Grow Lights and When to Turn Off

Turn your grow lights on when natural light alone isn't enough to keep your plants healthy and growing at the right pace. For most indoor setups, that means running lights 12 to 16 hours per day, starting them in the morning alongside (or in place of) natural sunrise, and shutting them off in the evening to give plants an uninterrupted dark period overnight. The exact schedule depends on what you're growing and what stage it's in, but those numbers are a solid, research-backed starting point you can use today.

Light schedules by plant type and growth stage

Not every plant wants the same number of light hours, and the growth stage matters just as much as the species. A tray of tomato seedlings has completely different needs than a mature pothos sitting on a shelf. Here's how to think about it.

The short-day plant category trips a lot of people up. Poinsettias, Kalanchoe, and Christmas cactus don't flower because they get less light total. They flower because the nights get longer and, critically, stay uninterrupted. Even a brief exposure to light mid-night can reset the flowering trigger. So if you're trying to get a poinsettia to bloom, the question isn't just when to turn lights on. It's also making sure nothing flicks on during those long dark hours, including a hallway light or a phone screen.

How to know when to actually turn them on

There are two ways to think about the timing of that first daily light-on moment: mirroring sunrise, or anchoring to a fixed clock. I prefer the clock approach, and here's why. Plants respond to consistent photoperiods, not perfect alignment with the sun. If you turn your lights on at 7 AM and off at 9 PM every single day, your plants benefit from that predictability more than they would from a schedule that drifts because you slept in.

A good rule of thumb for timing the on-period: start lights in the morning (anywhere from 6 to 8 AM works well) and let them run through the day into early evening. If you are wondering exactly when to turn on grow lights each day, start in the morning when natural light is low and keep the timing consistent when to turn on grow lights for indoor plants. Avoid running lights deep into the night if you can, since keeping a natural-feeling light/dark rhythm tends to reduce stress. That said, if your setup requires running lights from noon to midnight to avoid interfering with other schedules, most plants will adapt fine as long as the total hours and dark period are right.

Beyond clock timing, watch your plants for cues. If you're wondering when to use LED grow lights, start by matching the schedule to your plant type and growth stage, then fine-tune it based on their responses. If seedlings are stretching tall and thin (getting leggy), they're not getting enough light per day, and you should add more hours before assuming the light itself is the problem. If leaves are cupping, bleaching near the center, or showing crispy edges, those are signs the lights may be too close or running too long. Timing and distance work together, and you can't fix an intensity problem just by cutting hours.

How many hours per day: where to start and how to adjust

Start conservatively and adjust up rather than going full blast from day one. Here's a practical framework: seed starting and young seedlings respond best at 16 hours per day. Herbs, leafy greens, and foliage plants do well at 12 to 14 hours. Flowering plants that need encouragement usually want 14 to 16 hours.

If you're using supplemental lighting to boost natural light in a dim winter window, a rough guide from University of Illinois Extension suggests about two hours of supplemental light for every one hour of natural light you're missing, which in a low-sun winter setup can push your total runtime to 14 or even 16 hours a day. In winter, a grow light can be worth it when natural daylight is too weak for healthy growth dim winter window.

If you're transitioning from winter low-light to spring, don't jump straight to maximum hours. A strategy shared by experienced home growers is to increase by about one hour per day per week. So if you've been running 12 hours through January and February, step up to 13 hours in early March, then 14 hours the following week, and so on. This gradual increase lets plants acclimate without stress, similar to how they'd respond to a natural lengthening of days.

When to turn grow lights off: last-light rules and dark period basics

The off time matters almost as much as the on time, especially for flowering plants. Most plants need at least 6 hours of uninterrupted darkness per day, and many do better with 8 to 10. Darkness isn't just a rest period. It's when a lot of important biochemistry happens, including the hormonal signaling that drives flowering in photoperiod-sensitive plants.

For non-flowering plants and seedlings, the main rule is simple: don't run lights 24 hours a day. Even though some growers push seedlings to 22 hours under very low-intensity fluorescent lights (a technique that works in narrow circumstances with the right fixture and distance), most home setups benefit from at least a couple of hours of darkness. Running lights continuously long-term stresses most plants and can cause leaf bleaching, tip burn, and slowed growth, the opposite of what you want.

For short-day plants specifically: turn lights off early enough to guarantee at least 12 to 14 uninterrupted hours of darkness if you want them to flower. If lights go on at 7 AM, turn them off no later than noon to hit a 12-hour dark window. Any stray light during that dark period can prevent flower formation entirely.

Distance, intensity, and timers: the other half of the timing equation

Here's something that surprises a lot of beginners: changing how many hours your lights run is only one way to adjust how much light your plants actually receive. The total daily light dose (often called DLI, or daily light integral) is a product of both intensity and duration.

Purdue’s home hydroponics guide explains that for leafy greens you can think in terms of DLI, generally aiming for about 12 to 17 moles of light per day and increasing light intensity and/or duration if growth is too slow DLI is a product of both intensity and duration.

A light running at higher intensity for 12 hours can deliver more usable light than a weak light running for 18 hours. Knowing this gives you more flexibility when troubleshooting.

As a concrete example: to hit a moderate DLI target for leafy greens, you could run a light delivering 300 micromoles per square meter per second for about 11 hours, or run a weaker 210 micromol light for 16 hours. Same result, different schedule. This is why it's worth knowing your light's output, even roughly, rather than just guessing based on hours alone.

Distance is the most hands-on variable. Fluorescent shop lights should hang within about one foot of seedlings to deliver enough intensity. LEDs vary, but most consumer grow light instructions specify a range of 6 to 24 inches depending on the plant stage, with seedlings typically needing closer placement than mature foliage plants. If you move a light farther away to avoid heat or burning, understand that you may need to add more hours to compensate for the reduced intensity. And if your plants are getting leggy despite long photoperiods, distance is usually the first thing to check.

Please use a timer. I can't stress this enough. Relying on yourself to flip a switch at the same time every day will eventually fail, and inconsistent photoperiods stress plants and, for photoperiod-sensitive species, can throw off flowering entirely. A basic plug-in mechanical timer costs a few dollars and solves the problem permanently. Set it once, check it occasionally, and let it run.

Troubleshooting: what your plants are telling you about your schedule

Plants give pretty clear feedback if you know what to look for. Here's a breakdown of the most common signals and what to actually do about them.

Signs your lights are on too little or are too far away

• Seedlings stretching tall and thin (leggy growth): usually a distance problem first, then a duration problem. Move the light closer before adding more hours.
• Slow growth or small leaves on plants that should be vigorous: try adding 2 hours per day for one week and see if the growth rate improves.
• Pale, washed-out leaf color on plants that should be deep green: low light over time. Increase hours or intensity, or both.
• Herbs and leafy greens producing slowly in winter: this is almost always a seasonal light deficit. Add supplemental hours to hit 14–16 total light hours per day.

Signs your lights are on too long or are too close

• Leaf bleaching or whitish patches near the center of leaves closest to the light: intensity is too high for that distance. Move the light farther away rather than cutting hours.
• Leaf curling, cupping, or edges turning crispy: heat or light stress, often from a fixture that's too close. Check distance first, then consider cutting 1–2 hours from the daily schedule.
• Short-day plants that refuse to flower despite good care: almost always a dark-period interruption. Audit your room for any stray light sources during the dark window, including timers on other devices, nightlights, and windows.
• Foliage houseplants showing tip burn or stunted new growth despite healthy lower leaves: this can indicate too many hours at high intensity. Cut back to 12 hours and reassess after two weeks.

When to re-evaluate your schedule

Give any schedule change at least one to two weeks before judging results. Plants respond slowly, and a change you make today may not show up visibly for 10 to 14 days. The exception is acute light burn, which can appear within 24 to 48 hours and should be addressed immediately by increasing distance.

For seasonal transitions, reassess your runtime as the seasons change: what works in a December window will almost certainly need adjustment by March, when natural light quality and quantity start climbing again. If you're also curious about the broader questions of when to use grow lights indoors throughout the year, or specifically whether grow lights are worth running through winter, those questions tie directly into how you set your seasonal schedules.

That context helps you dial in the right daily schedule for your plants as daylight shifts across the seasons when to use grow lights indoors throughout the year.