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How many hours, and why the lamp needs a night
Plants keep time with the length of the night as well as the brightness of the day. A lamp that never goes off is not a kinder lamp.
A dark period is part of the setup
Leave an uninterrupted night. Do not run a grow light for 24 hours. A simple household pattern is to let the lamp define a day of roughly 10 to 16 hours, depending on what you are growing, and then switch it off.
The RHS page on how plants sense the environment uses commercial chrysanthemums as the illustration: growers lengthen or shorten the night on purpose because flowering responds to it. Your monstera is not a chrysanthemum, but the underlying point stands. Night is not wasted time.
What "enough hours" looks like at home
The RHS artificial lighting guide describes a basic home setup (two 60 cm T5 tubes over a small group of houseplants or winter salads) with the lights on for around 16 hours a day, and says to avoid running a system longer than the plants need.
That 16-hour example is a dedicated setup, where the lamp is the daylight. Use it as the upper end, not as a quota for every pot:
- Foliage houseplants kept as a winter top-up beside a window often sit in a 10–14 hour lamp day. If the sill is already bright, the lamp can run for less, because the window is doing part of the job.
- Kitchen herbs, especially basil, are greedier. A 12–16 hour day is the usual indoor schedule. The herb notes are in grow lights for kitchen herbs.
- Seedlings and cuttings are closer to the RHS 16-hour example: 14–16 hours, with the lamp the main light. See propagation lights.
The shelf tool picks the top of the range when you say daylight is low, and the middle when you say it is medium. That is a planning assumption, not a measured DLI. The arithmetic behind daily totals is in PPFD and DLI.
Use one timer, and know which lamps already have one
A plug-in timer is the reliable way to keep the day the same length on a Tuesday as on a Sunday. Put it on the socket, not on a second timer stacked behind a lamp that already times itself.
The SANSI 10 W clip gooseneck is an example of a lamp with its own timer. The maker states 3, 6 or 12 hour cycles, and says the lamp is incompatible with smart plugs and external timers. The same listing says the timer needs resetting after a power cut.
We haven't tested this kit. Specifications are as the maker states them (checked 13 November 2026).
Mechanical segment timers lose nothing in a power cut because there is nothing to remember, but they drift. A digital timer with a battery keeps the programme and then needs that battery. Either is fine. Two timers on one lead are not.
Cable routing and not overloading an extension lead are covered in timers, cables and shelf layout.