“Bedtime reading with a tablet or a smartphone can interfere with a goodnight’s sleep, some studies and many anecdotal reports suggest. Now researchers have conducted a small experiment to test the idea.” https://www.nytimes.com/2018/05/23/well/mind/how-nighttime-tablet-and-phone-use-disturbs-sleep.html
There is no point in using an iPad for reading books. It is much safer to use an inexpensive e-ink enabled screen such as an Amazon Kindle. Use an external source of light (not the white led one, but the hot orange one) and of course ignore the optional backlight available on some models.