Is this Temp O.K.?

My grow room is in my unfinished, unheated basment, I have the light on 20 hours off for 4, The air temp gets down to about 52 degrees and the soil gets to about 60, but only for the last hour and a half of darkness between 1:30 and three pm. My light come on at 3:00pm and by 3:15 its back up to 70-75. I know that in a month it will be much warmer around here (New England) and I won't have to worrie about it. The plants (ak47) are growing well, they are about 6 inches and only 3 weeks old. Should I not worrie about this. Or should I get a heater for the room? My electric bill is already much higher then I want to pay.

No more than a 15 degree swing in temp is ideal. But i wouldnt worry about it if it doesnt get below 50.

my recommendation is that an extra 4 hours of light isn't going to make a sweeping impact on your electric bill and considering the temp swing may be worth it.... if you're in new england like me you won't have an issue of it in a month or two depending on when you want to start flowering because it will be warm as hell. make sure you have a dehumidifier for any indoor grow for the summer months in this region. Your basement is likely to get the brunt of the humidity and can potentially cause mold/mildew on your goods.