Possible to vary light "on time" during veg between 18 and 24 hours?

I am veg'ing right now in a shed on an 18/6 light cycle (one week in so far). The weather is getting kind of cold right now, so I was thinking of maybe leaving the lights on for the plants to keep the temp up for them because when the lights shut off for those 6 hours, it may get a little chilly for them. So would it be ok to just keep the lights on for those 6 hours, even though the plants have already been on 18/6?

Should have no problems switching to 24/0

is it ok to switch between the amount of "on time" you give them? For example, could I give them 18 hours of light on and 6 off one day, and the next day give them 21 hours on and 3 hours off, then the next day give them 20 hours on and 4 hours off, then the next day give them 23 hours on and 1 hour off, etc...? (Not that i'm gonna do this, but is there anything wrong with it?)

Just go right to 24. But what will you do when you need to flower them?

Hey, just to say, having an inconsitant timing like that example where one day is shorter than another may not cause you massive problem, but will slow down growth or worse. And veg stage is very important.
if your meaning start seedlings off under 18hr, wait till they r mature and crank up to 24hr,as a more natural way maybe,, then go for 12/12 flower, and you could then decrease this to help ripen the buds.

Funny i actually read a good thing on plant circadian rhythms so it may be good reading..
https://www.icmag.com/ic/showthread.php?t=247234

So yeah, pick a schedule and stick to it, and get a timer. Reduced stress for fast growth..

Take it easy.

I could be mistaken, but I don't think bouncing around the light cycle during veg is going to do much harm. it's during early flowering when you really don't want to do that... if heat is an issue then definitely turn the light on 24/7 - but as jjoe75 points out - what happens when they are in 12 hours of no heat from yer lamp? you'll need a small heater to keep the tempy from dropping too much.