I (28F) have a girlfriend (34F) who won't stop staring at the sky. How do I get her to stop?

I'm at my wits end and I don't know what to do. I'm a little freaked out as well. My girlfriend is very into astronomy and she absolutely LOVES stargazing. We live in a spot with very little light pollution, so we can use a telescope to look at the stars together. We have paintings of the constellations everywhere in our house. sometimes I'd wake up in the middle of the night and I'd find her missing from bed, then go downstairs to see her out on the porch staring at the sky.

Sometimes she has insomnia or nightmares and she needs to calm down before bed, but recently she's been out there for a concerning amount of time. A month ago I woke up to see her missing from bed at 1:13am, so I went downstairs and kept her company. I kept her company for a bit then went to bed, because she insisted on staying down there alone for a bit. 

Well I woke up to go to the bathroom at 6:05am ish an she was STILL THERE on the porch. Same position. No expression, no change of movement, nothing. I asked if she was okay and got her something hot to drink but she didn’t want to talk about why she was out there so long. 

This has been happening for the past 2 months and a half or so. She’s a really private person so she’d definitely be upset to know that I’m posting this but I really don’t know what to do. I’m scared she’s going to be sick at this point because it’s been so cold lately. I don’t know when she’s sleeping. I’ve asked her to go inside, I’ve put blankets on her, I’ve literally stayed outside with her in a sleeping bag on some nights for solidarity. And it’s not like she’s hurting herself by staying out there but she just stares without blinking at the sky. Sometimes she won’t even answer anything I say. The only way I know she isn’t sleep walking is because she sketches while staring at the sky sometimes and seems to deliberately pick colors for her drawings, but she doesn’t even look down at the page. 

Any advice would be very, very appreciated, thanks for the help everyone.