Question: I have been having disturbing nightmares. What is the simplest Buddhist chant for peaceful sleep?
Answer: You should practise sincere mindfulness of Amitābha Buddha’s name (Āmítuófó: 阿弥陀佛), as can be learnt at https://purelanders.com/2016/02/17/how-do-i-start-nianfo-practice. For a start, this guiding track for mindfulness of the Buddha’s name (念佛) can be played for reference: https://purelanders.com/mp3.
Although short, the name of Āmítuófó is a supreme mantra, that contains the essence of the Buddhas’ teachings and blessings, as explained by the Buddha at https://purelanders.com/2011/12/16/the-name-of-amituofo-is-the-supreme-mantra. With it recited sincerely, evenly if mentally and in dream states, there will be instant protection, with any nightmare thereby ending.
For increasing your confidence, as you can see in these cases, this will surely work: https://purelanders.com/?s=nightmare. Other than practising when fresh and awake in everyday life, you should also practise mentally till you fall asleep. Although this will decrease the probability of having bad dreams, it will also increase your readiness to instinctively recite if entering a dream.