For the best reach, your bedside table should ideally sit 5-10 cm below the top...

https://amumreviews.co.uk/should-bedside-tables-be-higher-than-the-bed/

For the best reach, your bedside table should ideally sit 5-10 cm below the top of your mattress. This makes grabbing a glass of water or your phone effortless while you are in bed