black screen on startup

My machine is an iMac, Retina 5K, 27-inch, 2017, Intel Core i5, operating Ventura 13.7. Every Sunday night I shut down the computer and reboot it on Monday. Last week, upon reboot half of the screen was the expected wallpaper, and the other half was black for a few seconds before returning to normal display. Upon boot-up today, the screen was black, but the desktop icons were there. After more than a few seconds, the rest of the wallpaper was restored along with the menu icons on the left side of the screen (where I prefer them). For the rest of the week, I typically let the computer sleep. Also: occasionally upon wake-up, a blue screen will appear for anywhere from a split second to two seconds before allowing the proper screen to appear. Any suggestions, please?