ELI5: How do people die peacefully in their sleep?

My father recently passed from cancer. He was at home in hospice. He had essentially stopped eating for close to a week and started consuming less and less liquids as well. A few hours before he passed he started the death rattle and we had called the hospice service to send a nurse. She arrived and took his vital signs, said he was very close to dying. About 10 minutes later she checked him again and he was gone. There was no noise, no thrashing about, he simply stopped being alive. He was on morphine and some anti-anxiety drugs to help keep him calm and pain away. I doubt he felt any pain when he passed. It would be accurate to say he died peacefully in his sleep.