My girlfriend's great grandmother passed away. She was 99 years old and died in her sleep. I can't think of a better way to go. How many times have you heard someone say that they'd like to die in their sleep when they're very, very old?
Sleep is a kind of rehearsal for death, the "big sleep". To die after a long life, and to pass from this existence on earth without much, if any, pain, is the way most of us would choose to exit life. And to die while sleeping, without any warning that your time is almost up (unlike when one faces a serious, life-threatening illness) is the ultimate path to immortality. Not the "immortality" of being famous, which is no immortality at all. To paraphrase Woody Allen, I don't want to live on through my work, I want to live on in my apartment. But when you don't know you're dead, when all you knew or would know is life and consciousness, having gone to bed with the reasonable expectation you would wake in the morning to start another day, you have achieved what all of us have now, eternal life. The eons before you were born mean nothing to you, and so will the eons after your "death". Your life is your consciousness, and YOU, the structures in your brain that create your personally, will always be you. You will never know anything but life. YOU cannot experience death. In this sense, the religious are right when they say that your body is not the real you, which it certainly isn't once you lose consciousness forever, though YOU won't be aware of when that happens.
What a lucky woman her great grandma is. We should all be so fortunate.