My girlfriend wants to kiss all the time?

Ok so me and my gf are 13 and in 8thgrade same school and we see each other Monday thru Friday at school and hang out all day on Sunday's. When I go to her house I feel so relaxed and sleepy cause I feel comfortable around her and not this way at my house. So when I get there we usually cuddle on the couch and it gets me so relaxed and sleepy so she puts her head on my shoulder and I'll put my head on her head and want to close my eyes to take a short like 30 min nap cause we hang out for like 9 hours. And she"ll always just take her head off my shoulder which causes my head to just fall and she does it just to kiss me all the time. And I've explained to her that when im with her I feel relaxed so I just want to take a 30 minute nap and sometimes when were snuggling in the way i described she'll remove her head from my shoulder and pucker her lips and look and me and poke my arm and me being tired will have my eyes closed cause I'm tired and have to open my eyes and kiss her all the time and the whole time Im there I'll be tired. How can I tell her I just wanna nap with her for like 30 min?