Most seedlings need 16 to 18 seedling light hours each day to grow strong and stay short. A few hours of weak light by a window will not cut it, and you will see the plants stretch and lean within days. Set a timer for that long stretch of bright light for seedlings and they stay stocky instead of thin and floppy.
The stretching happens fast. Your seedlings reach toward whatever light they can find, so a dim windowsill makes them grow tall and weak as they hunt for more. Those long, pale stems can't hold the plant up later, and you end up with leggy growth that flops over. A bright lamp running most of the day gives them no reason to stretch. They put that energy into thick stems and real leaves instead. You want short and stocky plants, and a long bright day is how you get them.
Both how long the light runs and how bright it is matter here. You can leave a lamp on all day, but if it sits too far away or puts out a weak glow, your seedlings still reach for it. Aim for a strong, even light that covers the whole tray. The right seedling photoperiod pairs that long run time with real brightness, not just a long stretch of dim light. Think of it as two dials you set together. You turn up the hours, and you turn up the brightness too. Get only one of them right and your plants still suffer.
Both Clemson and the University of Minnesota point to the same range. They both say 16 to 18 hours of light a day for seedlings started indoors. That range works for tomatoes, peppers, and most of the flowers and vegetables you grow from seed. It gives your plants far more light than a short winter day ever could. That gap is the whole reason your indoor seedlings beat ones stuck on a cold windowsill. You control the day length, so you give them a long one.
Seedlings still need a dark period each day, so don't run lights 24 hours. Plants do real work in the dark, and a 6 to 8 hour night helps them grow steady and strong.
Skip the round-the-clock lighting even though it sounds like a fast track. Your plants rest and process their day's growth at night, much like you do. A solid 6 to 8 hour dark window each day keeps that rhythm intact. More light past 18 hours gives you little extra growth, and it just runs up your power bill. So you gain nothing and pay for it. Stick with the right seedling light hours and a proper night, and your seedlings stay on the right track.
A cheap plug-in timer makes all of this simple. Set it once for an 18 hour on, 6 hour off cycle and walk away, because the timer handles the schedule every single day without you. You won't forget to flip the lights on before work or off at night, and the seedlings get the same long day whether you're home or not.
Pick brightness over a sunny window every time. A south-facing window in spring still gives you maybe 8 to 10 hours of usable light, and weak light at that. A simple shop light or grow lamp on a timer beats it by a wide margin. You control the hours, and you control the strength, so you don't depend on the weather. Get those hours right first. Then you can sort out how close to hang the lamp, since that distance is its own question with its own answer.
Read the full article: How to Fix Leggy Seedlings: 6 Proven Steps
