There is NO nighttime feature or mode on the device at all. If you want the nighttime feature, you will need to get the Barnes & Noble Nook Glowlight 3 e-reader.
Also the closest nighttime feature on the device is called the inversion of from white with black letters to black with white letters. You can go to the 3 vertical dots on the right side on the Home Screen. Tap on the dots. Then you will see the word "Settings". Tap on the word "Settings". Then you will see the word "Accessibility". Tap on the word "Accessibility". There you will see that you can swipe the toggle switch to change the color of the e-book page from black to white or white to black.
Please Note: That if you change it from white to black, ALL screens on the e-reader, and that includes the Home Screen and Settings Menu, etc. and the e-book page inside of your e-book, will be ALL changed to black with white letters.