temperature at night/lighting question

got my seedlings on a 18 hour cycle...

but at night time the temp in my closet dips down to the lower 60s...

I sent a pic to my friend in the NL and he said that my stems looked reddish, so that means overwatering, not enough nitrogen, or the temp is dipping below 66 degrees...

the temp is definitely dipping below...

here is my question

should I buy an automatic time for a space heater and put it in there to turn on after the lights go out? OR should I change the light cycle from 18 hours to 24 hours?

will this mess my seedlings up? this would be the easiest method (changing it to 24 hours that is)

thanks guys and gals!

Johnny

The buds don't mind the lower temps like the vegetation sometimes does, ie: slowing to a crawl.
If 66* is the line, I'm screwed. Seems a bit high to me. My plants get lazy <50*.

Is it your lights that are making the difference in temp? If so, I'd go 24, for the heat if nothing else. I HAVE to run 24 just to stay in the 60's when it gets cold outside.
Little slower, but no real other negatives.

Lower 60s wont hurt ur plants, my temps range low 60 , lite off ,to upper 70s, lite on. During cold I turn on heater to keep 60s, at nite, especially during flowering. Red or purple stems normal, just showing Indica strain. Keep On Growin