is 15c night temp 2cold for seedlings?

hey guys.. well i had heat probs slightly.. now its getting really cold.. my day timew room temp is like 25c.. n just above my 10-11 day old seedlings temps bout 28-29c.. with fans nice and cool.. well lights are on 18/6 from 5am-11pm.. so dark times from11pm till 5am.. i checked the temp at bout 12am lastnight temp was 15c.. and 1 baby has turned slightly purple round the edges of the baby leaves... will my babys be ok at that cold temp 15c for 6hours every night?? if not would closing my window help much?? coz the tent is right next to the window... coz i will be pissed if all my babys die... i mean they all look nice n green a little droppy but its all jiggy.. im just worried about them night time temps.. but im hoping some1 with more winter growing experiance might tell me i have nothing to worry about and will be ok.. wish my blueberry n aks luck.... im gonna go sing to them now;)
pps.. i know i can go 24/0 and have no cold weather problems.. but i will not do that as i have read that 18/6 is better gives the plants some rest time especilly if its hot during the day.. allso ive heard 24/0 can cause males or hermies.. ive never gone wrrong on 18/6 in 3 grows so im not gonna change now..

Switch your lights to even out the temps... it's colder at night, so have your lights OFF during the warmest part of the day (11AM-6PM or so).
Or put in a heater.
Even if it doesn't kill them, cold wet soil amplifies nutrient problems and stunts growth.

hey stinky.. well i just thought about what you said.. is there a chance that even if i switch lights off at say 12lunch time till6pm that it could still be too cold in there for them? as the days are very cold at the moment and getting colder.. hense would i just be better off putting a little heater in there with a timer so it kicks in lights out n turns off lights on? i received a magazine from my local hyro store today.. they have heaters in there for grow rooms at £16.99... 2kw fan heater three settings 1000w 2000w and cool blow does that sound ok? thanks again stinky;)

I don't know how fast your growroom loses heat...so you are going to have to experiment with your setup.
My lights go off at 0830 and come on at 2030 in flower because I have less control over temps than I'd like... but the seedlings and veg are on 24h and I don't worry about m/f ratios, just survival, lol!
Ask again in winter... I may just say to hell with it and move back into the cellar growroom that holds heat better, and tell my tenant to just suck it up and deal!

after another think.. i thought i might go with 20/4 on the lights for veg.. so they only have 4 hours in the cold.. n just b4 i flower them i need to go to my local hydro store for perlite light hangers and such so i think i will pick up a little heater then.. so when i flower them i can have the heater on the 12hour dark period so they aint cold.. i hope with a bit of love they will be ok in the cold for 4 hours a day.. then when i flower them i bet there will be no stopping them hehe..

i just had another thought... i dont want to risk my babys getting too hot with a heater lol.. but i remember reading about heating pads for seedlings ... just cant remember where... can any1 who has had experiance using heating pads for there seedlings let me know if they are any good?? thanks all spliff time for me now i think

I USE CERAMIC HEATERS, FROM WAL-MART. $12.

AND THE ARE DIGITAL. SET UR LOW TEMP, AND IT WILL RUN WHEN IT GETS THAT COLD.

MINE RUN ABOUT 3 DEGREES HIGH AND/OR LOW.

VERY CHEAP INSURANCE FOR YUOUR BABIES