It is only okay if she is okay with it in advance. Otherwise it is not only wrong but illegal. Most states would put that on the same level as rape. Personally I love waking up to sex and Ive initiated it on other willing partners however it must be okay with all persons involved.
Talk to your girlfriend about your desire. Give her permission to initiate sex while you are sleeping and see if she is willing to do the same for you. If and only if she is clearly okay with it are you okay to have sex with her while she is sleeping. In fact it wouldn’t hurt to discuss how you can hold her in her sleep such as cupping her breasts or with your leg against her lady parts. Otherwise she could claim you assaulted her while she was sleeping.
In today’s world it might not hurt to have it all in writing so it doesnt come back to haunt you later. She could claim there was never any understanding. Having it spelled out on paper would prevent any misunderstandings or you finding yourself in trouble even years later. I hope it works out for you. Good luck.