Cannabis Light Cycles Explained: 18/6, 12/12, and When to Switch

Green cannabis plant in the vegetative stage growing under an LED panel light indoors — Indoors, your lamp and timer play the role of the sun — and the seasons.

Cannabis is a photoperiod plant: it decides when to grow and when to flower based on how many hours of uninterrupted darkness it receives. That single fact explains most of what indoor growers do with their lights — and most of what goes wrong when a grow stalls or flowers too early.

This guide explains the standard light cycles, what actually happens inside the plant when you change them, and how to avoid the light-related mistakes that cost beginners entire harvests.

Why darkness, not light, controls flowering

It feels intuitive that more light equals more growth, but the flowering switch works the other way around. Cannabis measures the length of its nights. When darkness stretches past roughly 12 continuous hours, hormonal changes tell the plant that autumn is coming — time to reproduce, which for growers means time to produce buds.

This is why indoor growers speak in ratios: 18/6 means eighteen hours of light and six of darkness, while 12/12 means an even split. It's also why a single light leak during the dark period can confuse a flowering plant badly enough to stress it back toward vegetative growth.

18/6 — the vegetative standard

During vegetative growth, most indoor growers run 18 hours of light and 6 of darkness. Long days keep the plant firmly in growth mode, building the roots, stems, and fan leaves that will later support heavy flowers.

Some growers run 20/4 or even 24/0 to push faster growth, but 18/6 remains the beginner-friendly default: plants get a genuine rest period, electricity costs stay manageable, and heat is easier to control.

Cannabis plants in the vegetative stage under bright strip lights inside a reflective grow room — Vegetative plants under long light hours — 18/6 keeps them building roots, stems, and leaves.

12/12 — the flowering trigger

Switching your timer to 12 hours on, 12 hours off is how indoor growers start flowering on demand. Within one to three weeks, plants show their sex and begin developing buds. From that point, the dark period is sacred: even brief interruptions from an unsealed tent zipper, an equipment LED, or an impatient peek with a flashlight can cause stress, delayed flowering, or hermaphroditism.

Outdoors, nature moves the switch for you — plants begin flowering as nights lengthen in late summer. That's also why growing outdoors requires planning around your local season rather than a timer.

Flowering cannabis plants developing buds under purple-red LED grow lights — After the flip to 12/12: buds form while the dark period stays strictly uninterrupted.

When should you switch?

There's no single right answer — plants roughly double in height during the flowering stretch, so the real question is how much finished height your space can handle. As a rule of thumb, beginners in a standard grow tent flip to 12/12 when plants reach about half of the height they can accommodate.

Time matters too: most growers vegetate indoor plants for 4–8 weeks. Shorter veg means smaller plants and faster harvests; longer veg means bigger yields per plant but more time, water, and risk.

Common light cycle mistakes

Watch for these — each one is easily avoided:

Light leaks during the dark period, the classic cause of stressed or hermaphroditic flowering plants.
Cheap timers that drift or fail — a stuck timer can silently re-veg a flowering crop.
Switching to 12/12 too late and running out of vertical space during the stretch.
Forgetting that autoflowering strains ignore photoperiods entirely — they flower on age, not darkness, and are usually run at 18/6 from seed to harvest.

Light cycles are one piece of a bigger system

Photoperiod interacts with everything else in your grow: light intensity and distance, nutrient demands that change at the flip, training techniques that have to finish before stretch begins. Understanding each piece in sequence — rather than patching knowledge together from forum threads — is the difference between a stressful first grow and a confident one.