nipples

Hello… Was just looking for some advice. I have two sphynx girls both who have just had their first season/ heat. My pink girl has circles around all of her nipples while my other girl has nothing at yeh moment. the are both coming up for a year old. The haven’t been around any un neutered...

So my little Ziva keeps sucking her nipples whenever she gets comfy. She could wake up at 2:00am and strat sucking. It is getting a bit annoying. I tried to discourage her but nothing works. I wanted to see if anyone else had this issue and what can be done to stop this behavior. Thank you