If you mean 6°C, this is not an ideal situation for a young, growing tortoise overnight, but you can bring it indoors overnight and keep it outside during the day if basking temperatures can reach 90°F (27-32°C). If they don’t get this warm, just bringing her outside for short periods (30-60 minutes) of time daily when the weather is near 27°C/80°F while you are around to protect her is fine.