Why not just tell them the truth? When you started it was supposed to be temporary but you so liked them all that you continued for 7 years longer than you planned. Now it's getting more and more difficult to commute the distance. (3 hours takes a big chunk out of your day!) The night shift is not agreeing with you anymore and it is getting harder and harder to stay awake when you are driving home. For your safety and health, it's time for you to find a day job. This is a very hard thing to do I know, and the family will not like starting over with a new nurse, but they will hopefully understand.
It's hard to tell a family after many years that you want a job change and need something more challenging. I found that having an 'excuse' is easier rather then saying you just need a change.
I've been in this situation twice and ending it went well with one, not so well with the other. The first was a little girl I had been with since she was a baby. (she is 12 now) They moved to a city an hour away. I continued to work once a week for awhile after they moved. My sister lived in the same town so I would work a 12 hr shift and spend the night at my sisters. I was a good way to spend time with her and it worked nicely for awhile. When Mom switched agencies and the weather got bad, I told her I couldn't continue the commute and didn't want to hire on at another agency. The last shift was sad, one of us was crying the entire time. But we have remained friends to this day and I still visit them.
The other child, I was with for 9 yrs. I started when she was only a few months old. She was a foster child and her original foster parents got a divorce. Dad retained custody of the child. He remarried and they adopted her. I was there for about 5 yrs after the divorce. Things got a bit dysfunctional in the household after that. I stuck around for the sake of the child but after a while I got to the point where I needed a change. I had some surgery and decided this was an ideal way to leave without hurting any feelings because lifting wasn't recommended afterwards. I said my goodbyes and all seemed well. Then the second wife started telling everyone terrible things about me. I was hurt and shocked. She even went as far as to tell the rest of the nurses on the case (who knew me well) that I was having an affair with her husband. I continue to be very hurt by this since absolutely none of what she said was true. I stayed there for so long because I thought I was making a difference in this child's life and that I was appreciated. I was instrumental in helping the Dad obtain custody of the child when he was clearly the better choice and his ex-wife was winning the custody battle. My testimony won him the case. There were other significant things that I had done during the years that helped this family a great deal, so when all this happened, it was shocking. I miss the child and hoped to stay in touch but under the circumstances, I have not done so.
I hope it goes well for you. But realize that the family may feel betrayed. Hopefully, they will be mature and react in a good way.
What I did learn in all this is that we are all dispensable. As bad as that sounds, I now know that another nurse can and will step in and do as well or better than I did. So staying because I thought I was needed or that the family would suffer without me is not the reason to stay.
Sorry, didn't mean to write a book here!