You can very safely leave your lights on the 18 hours on, 6 hours off cycle.

The rest of the seeds will germinate and grow fine.

Agree. 18 on 6 off works fine.

Dave

I leave mine on 24 hours for the first week. After the first week I then set my timer for 16 to 18 hours on.
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I seem to get a slightly higher proportion of seeds emerging at night so the dark period may even be more beneficial at that stage of growth, and it certainly helps with the humidity (warm light dry the air). The plants are mainly working off of stored food at that stage so a few less hours of light isn't a bad thing.

I agree 100% with spiced ham. I also use the 18/6 light cycle and note remarkable activity at the conclusion of the 6 hour off cycle. This starts on either the 3rd or 4th day after planting.

Plants are pretty much dependant on and are in tune with the
cycles of nature so their response to a day/night cycle might
logically be anticipated.

I'll give them their 6 hour sleep each day then. Thanks!

I tried the 24 hour light thing over the winter on a tomato I was overwintering in a five gallon bucket. It seemed to be static and almost sickly - not doing much. After a few weeks, I started on the 18/6 schedule and the plant had a major growth spurt and quickly set 4 blooms given us fresh Christmas tomatoes.

It seemed like the plant needed the dark time to process all the energy it had gathered during the light time - as though a switch turned off and on depending whether it was light or dark. Anyhow, I used the same schedule on my germination trays after sprouts showed and have beautiful healthy transplants, some of which will go into their containers beginning today.

Ted