Feel guilty about resigning during the holidays

I work at a rehab/snf. I have been there for 7 months right out of school. I have found it is not for me, I work nights and cannot get a day/eve shift position and I am not adjusting to nights at all. My whole life is affected by my inability to sleep properly and I can feel it affect my overall health. Not to mention, I have been offered my dream job as a pediatric RN and I am just thrilled. I will be handing in my letter of resignation in the next day or two, and I feel really guilty because it is holiday season (though I ended up not having to work any holiday this holiday season). I am giving them 3.5 weeks until my last day. Nobody ever wants to cover night shifts and I really just hope this is enough time to find someone. Any advice on how to make this go over as well as possible when I hand in my letter to the DON? I have considered offering being on call until they find someone since I am full time but deep down I just want to focus on my new, full time job...Thank you
If I were you, I'd hand it in and keep any explanations simple: your sentence (above) about your health being affected should be sufficient. Don't feel guilty, and fer gawd's sake don't volunteer for any additional time to help them out. It's THEIR job to staff the facility, not yours. Plus it would negate your reason for leaving if you're willing to stay on to 'help out' in spite of how it is affecting you.
Either you resign and move on, or you stay on in a part time or on call status, or you just plain stay. Make a decision and stand by it or you will find yourself manipulated and taken advantage of to your detriment.
Do not feel guilty. I am sure you worked hard for every paycheck. Give the required 2 or 3 weeks notice. Write a brief professionally worded letter of resignation. Try to leave on a good note. If they ask you to stay on per diem, say I can not commit to any extra shifts because my new position is full time. It's just business. They can hire a travel nurse or an agency nurse with a few phone calls. Do not feel guilty or indebted.
Don't worry too much about leaving your employer high & dry. It's the hospital's place to make staffing. And you can bet that if they choose to terminate someone during the holidays, they will do so!!!Like others have said, keep your resignation short & sweet. I wouldn't go into any detail at all. Employers usually keep resignation letters in your file and you wouldn't want it to come back and bite you (like your current place telling new place you didn't like 11-7 and that's what is being offered to you at new place). And yes, employers can give information to new employers as long as it is the truth. So, don't give them the ammunition in your resignation letter.Just tell them 'thank you', that you learned a lot, worked with great people, yadda, yadda, yadda. And move on.