The ideal range is between 74°F and 78°F, this is where enzymatic activity runs at its most efficient. The bottom end should be no lower than 68°F or higher than 86°F.
Although the plant can survive well above and below these thresholds, the plants efficiency for photosynthesis reduces dramatacally outside these ranges.
You ideally dont want the temperature difference to be any more than 50°F.
I decided to grow this winter, with ambient daily temperatures around 55°F and the lows as far down as 26°F. I feed my aircooled 600w light air back to the bottom of my tent and use fans to keep the hot air from rising. I also have a intake to bring fresh co2 into the tent and even with this, my light can keep a temperature difference of 71°F. This means that i can handle down to 35°F to keep internal temps at 75°F before a additional heater is required, for which i use a 1000 watt oil heater on a thermostat.
When the lights off, the heater handles the rest.